[Logger] Set log level (severity)

[marceloboeira]

This is already a Crystal::Logger feature, to define the severity, but we must provide a public API in order to define the severity of the log output.

e.g.:
bojack server --log-level debug || bojack server --log-level 0
bojack server --log-level info || bojack server --log-level 1
bojack server --log-level warn || bojack server --log-level 2
bojack server --log-level error || bojack server --log-level 3
bojack server --log-level fatal || bojack server --log-level 4

Reference: https://crystal-lang.org/api/0.18.7/Logger/Severity.html