Add --output-style option to repoman

This patch adds a --output-style option to repoman, which gives the user
a choice of output formats for the repoman checks. Choices are "default"
(current style) and "column" (a greppable format), but it should be easy
to add more. Fixes bug 481584.

v2: Fix docstring to be complete and in the standard format, make use of
default choices in --output-style wrt comments by antarus and dol-sen
2 files changed