Genbadge

- Completed CLI help for subcommands. Fixes [5](https://github.com/smarie/python-genbadge/issues/5)
- Badges can now be redirected to `stdout` using `genbadge <cmd> -o -`. Fixes [4](https://github.com/smarie/python-genbadge/issues/4)

- New command `genbadge coverage` able to generate a badge from a `coverage.xml` coverage report

- Command group `genbadge` with global help
- Command `genbadge tests` able to generate a badge from a `junit.xml` tests report, with :

- color depending on success percentage (50%/75%/90%)
- customization of input `junit.xml` file and output SVG badge file,
- custom source (`shields.io` by default or local SVG file template for offline usage).
- "fail on threshold" option to return an error code 1 when the success percentage is strictly lower than the threshold.