Generate a "git stats" page

[iphydf]

This should be a new tool and also be available through the web api.

Original idea proposed by @zetok in TokTok/c-toxcore#351:

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[Proposal] Automatically generated & deployed to gh-pages gitstats

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Default CSS might not look very good, but it is a nice way to show that "stuff is happening".

e.g. https://qtox.github.io/gitstats/authors.html