Add envconfig to project

[rafaelhenrique]

Trying solve issue #6 :smile:

[kshitij10496]

This looks good @rafaelhenrique 😄 🎉

Just a few suggestions to make this ready for merge:

1. As @hypnoglow mentioned above, we can provide a required flag to enforce that both the variables are set. 👏
2. Use a default value for PORT(say 8080) which can be simply set using the default tag.
3. I would appreciate it very much if you can add a description using the desc tag for each of the variables.
4. Define the Config struct in common package and import it in server.go.
5. Process the input as:

   if err := envconfig.Process("hercules", &config); err != nil {
       fmt.Fprintln(os.Stderr, err)
       envconfig.Usage("hercules", &config)
       os.Exit(1)
   }

[rafaelhenrique]

Applied changes @kshitij10496 @hypnoglow ... thanks for your tips :wink:

[kshitij10496]

Great work @rafaelhenrique! 🎉

I will wait for @hypnoglow's review before merging it in. 😄

[kshitij10496]

Thanks, @rafaelhenrique, and @hypnoglow for your invaluable contributions! 🎉 🤗

[rafaelhenrique]

:tada:

[kshitij10496]

UPDATE:

I deployed the changes to Heroku only to later realise that the platform sets the PORT environment variable for each application. What this means is that it won't update the new HERCULES_PORT variable. Thus, I had to force the envconfig to recognize PORT and read it's value into config.Port variable. 😅 I will try to be more aware of these subtleties in the future.