Refactor change_viz in Formatter

Do not pass the whole pull request as argument, use additions and
deletions as params instead. Allows for better reusability.
Refactor in Pull::Show

DRY DRY DRY
Fix typo in change_viz of Formatter
Show files in Pull::Show through option -f

Formatting refactor pending.
Better formatting in Formatting#format_file
Extract constants in Formatting

Extract colorcode dictionaries that don't need to be assigned with
every method call.