Allow to configure the log verbosity level

[djhi]

Problem

The current output is quite verbose even without the --verbose flag. This pollutes CI logs with unnecessary lines.

Solution

• Introduce a --verbosity argument that can be verbose, normal or silent. It takes precedence over the --verbose flag if both are set.
• --verbose is still supported and used if --verbosity is not set
• Keep error and warnings logs even in silent mode

Fixes #49