Automatically wrap commit body

[sonnyp]

Fixes #52

Defaults to 72 as it seems to be the most popular recommendation. GNOME suggests 75 but 72 will work too.

Here is the rational torvalds/linux#17 (comment) - the fact that we automatically wrap will cause issues with the following

Some things should not be word-wrapped. They may be some kind of
quoted text - long compiler error messages, oops reports, whatever.

But we can add a mode that disable the word wrapping and add a ruler. #58