[Feature request] Add discussion in code and commit diffs #60

It would be nice to be able to discuss a change (like on Github) and a line (in the code viewer, by simply having some button like "Start discussion" at the side).

In the latter case, it would be nice to notify (via email or alike) the user who wrote the code in the first place (say à la "git blame") via email for instance.

The discussion thread could be shown at the side of the code (at the left) and hidden by default, showing only a small icon or someway highlighting the affected LoC