Add chart "commits_day" and option -s/--sort-max

New chart "commits_day" shows commits by day (by default the 20 last
days with at least one commit).

Two charts have been renamed:
- commits_hour -> commits_hour_day
- commits_day -> commits_day_week

New command line option -s/--sort-max: keep max entries in chart and
sort them by value.

Option -m/--max has been renamed to -d/--max-diff and is now used as
well for the new chart "commits_day".
Ignore UTF-8 decoding errors

The script was throwing an error when building chart "authors" with
Linux git repository (some names are not UTF-8 valid). The decoding
errors are now ignored.
Fix parsing of authors

The script was throwing an error when building chart "authors" with
Linux git repository: some lines with author have a count, but no name.