Gitchart

- Add option `-j` / `--js` to customize javascript files/links.
- Fix the working directory for the second git command.

- Fix PEP8 warnings.

- Fix parsing of authors.

- Ignore UTF-8 decoding errors.

- Add chart "commits_day".
- Rename chart `commits_hour` to `commits_hour_day` and `commits_day` to `commits_day_week`.
- Add option `-s` / `--sort-max`.
- Rename option `-m` / `--max` to `-d` / `--max-diff`.

- Fix chart title.