Add basic support for NPRE standard exit codes

Add exit code table and description to the README
Also some automatic whitespace striping in README due to editor
Add NPRE exit codes to the artisan commands which other monitoring tools utilise
Kept current behaviour for OK/FAIL but reworked it to allow full support of the varying levels eventually
Add filtering to the /health/string endpoint to only show the results the caller is interested in
Adjust /health/string handling, adding more options but keeping backwards support for ok/fail messages.
Add format option to the /health/resources/:resource endpoint to show an alternative format to json, with a summary of the service in plaintext
Service status updated to return the highest level of severity (starting from OK == 0)

Example output

root@a01fe36051f6:/siteroot/api# php artisan health:panel
+--------------------------------------+----------+--------------------------------------------------------------+
| Resource                             | State    | Message                                                      |
+--------------------------------------+----------+--------------------------------------------------------------+
| App Key (default)                    | OK       |                                                              |
| Cache (default)                      | OK       |                                                              |
| Configuration Cached (default)       | Critical | Configuration is not cached                                  |
| Debug Mode (default)                 | Critical | Application is in debug mode                                 |
| Directory Permissions (default)      | OK       |                                                              |
| Disk Space (root)                    | OK       |                                                              |
| .env Exists (default)                | OK       |                                                              |
| Filesystem (default)                 | OK       |                                                              |
| Framework (default)                  | OK       |                                                              |
| LaravelServices (default)            | OK       |                                                              |
| LocalStorage (default)               | OK       |                                                              |
| Migrations Up to Date (default)      | OK       |                                                              |
| NginxServer (default)                | OK       |                                                              |
| Php (default)                        | OK       |                                                              |
| Queue (default)                      | OK       |                                                              |
| QueueWorkers (default)               | OK       |                                                              |
| RebootRequired (default)             | OK       |                                                              |
| Routes Cached (default)              | Critical | Routes are not cached                                        |
| ServerUptime (default)               | OK       | Looks like your server was recently rebooted, current uptime |
|                                      |          | is now "2 weeks 17 hours 56 minutes" and it was "1 week 6    |
|                                      |          | days 21 hours 4 minutes" before restart.                     |
| Supervisor (default)                 | OK       |                                                              |
+--------------------------------------+----------+--------------------------------------------------------------+
root@a01fe36051f6:/siteroot/api# echo $? # print out exit code
2

https://localhost/health/resources/queue?format=summary

OK: Queue is running as expected (Checked Sep 8 16:09:01)

https://localhost/health/resources/configuration-cached?format=summary

CRITICAL: Configuration is not cached (Checked Sep 8 16:09:27)
PragmaRX\Health\Checkers\Expression was used to determine the health

antonioribeiro / health

Add basic support for NPRE standard exit codes #241