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

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.

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".

Make options title/repository optional, add option -m/--max, major code cleanup, full PEP8 compliance

All changes:
- chart title and repository are now optional and must be set with two
  new options:
    -t, --title (default: hardcoded in script, depends on the chart)
    -r, --repo (default: '.', ie current directory)
- add option -m/--max: max different entries in chart: after this
  number, an entry is counted in "others" (only for charts "authors" and
  "files_type")
- if filename is "-", display SVG content on standard output
- import the new division operator and the print function
- replace %-formatting with .format()
- full PEP8 compliance
- major code cleanup: add comments, dynamically build the names for week
  days and months, use a dynamic name for functions building a chart.

Add PNG support