main: add toggle for commit title overflow

Hi Jonas,

This PR is the v3 of #122, which now implements the new **title-overflow** option as a boolint.
That means, we can set the title overflow highlighting like this:

    set title-overflow = yes # defaults to 50 character
    set title-overflow = false # disables highlighting
    set title-overflow = 72 # enables and sets the limit to 72 character

Cheers,
Vivien