Add a daemon mode for container checking

[josegonzalez]

Description of feature

In daemon mode, the container(s) would be checked according to the settings of each healthcheck. Some notes:

• --filter: check containers by docker container filters (updating as containers start/stop). This should also be added to the check command.
• A new on_failure parameter for healthchecks:
  • command: Runs a command on the host with data about the healthcheck and the failing container.
  • url: Sends the failed check data to the specified url
  • on_nonzero_exit: Whether to trigger the on_failure step if the watched container has a non-zero exit

[josegonzalez]

Added #44 to track config-less healthcheck definitions.

[josegonzalez]

Okay now we just need a daemon!