Make Prommer configurable with a configuration file

[tomverelst]

Prommer currently only supports command line flags. Configuration must be able to be stored in a configuration file and passed on as a command line argument. Example:

$ prommer --config-file <path-to-config-file>

Possible formats include JSON and YAML, but since Prometheus uses YAML, preference goes to YAML.

Tasks:

• Define format and possible settings
• Update README with example configuration file