Add per-repo user-specific diff statistics to graph output

[alecello]

A new graph is generated in each one of the per-repo figure with a bar graph containing the all-time and yearly diff statistics for each contributor for that specific repo.

The new graph is a stacked bar graph with two colors, the familiar green and red for added and removed lines.

[alecello]

Should close #7

[e-caste]

I see 3 issues in an example diff histogram here:

• one name does not display correctly
• many numbers are written on top of each other when the bars are short
• of the 3 numbers (deleted, added, total) I can only see the number of deleted lines and the total

Could you address these issues?

[alecello]

Regarding point 3: There is no total, just deleted and created lines

[e-caste]

@alecello I believe it is not clear then. Best way is to change how the data is displayed, a number in the middle of the bar is not very readable

[alecello]

Ok, it's going to take a lot more time than anticipated. Since I can't complete this right now (needs to rewrite a bunch of code), I'm marking this as draft until ready