main: add options for short commit titles

This PR is the v2 of #122. Give it a look for details and see [what it looks like](https://a248.e.akamai.net/camo.github.com/0e3696fbe440544ab46da6dfca05faf10c513ceb/687474703a2f2f692e696d6775722e636f6d2f644e524b3546772e706e67).

The patch adds the ability to highlight text exceeding a specified
width in commit titles, through a "show-short-title" toggle.

It also adds a "short-title-width" option to change the maximum value to
consider a commit title to be short. This value defaults to 50.

Best,
Vivien