Move all logging verbosity options to a single argument --loglevel

[blahah]

Implement the same as in transrate. Relevant code:

• https://github.com/Blahah/transrate/blob/master/bin/transrate#L13-L28
• https://github.com/Blahah/transrate/blob/master/bin/transrate#L80-L81
• https://github.com/Blahah/transrate/blob/master/bin/transrate#L164-L167
• https://github.com/Blahah/transrate/blob/master/lib/transrate.rb#L15-L25