Maintainer count: add frontend

This adds support to bin/count-maintainers for the Frontend Developer / Maintainer ratio:

Before:

| Date | Maintainers | Developers | Developers per maintainer |
| --- | --- | --- | --- |
| 2016-07-01 | 5 | 26 | 5.2 |
| 2017-01-01 | 6 | 25 | 4.2 |
| 2017-07-01 | 7 | 29 | 4.1 |
| 2018-01-01 | 8 | 31 | 3.9 |
| 2018-07-01 | 8 | 47 | 5.9 |
| 2019-01-01 | 11 | 54 | 4.9 |
| 2019-04-04 | 13 | 72 | 5.5 |

After:

Backend:
| Date | Maintainers | Developers | Developers per maintainer |
| --- | --- | --- | --- |
| 2016-07-01 | 5 | 26 | 5.2 |
| 2017-01-01 | 6 | 25 | 4.2 |
| 2017-07-01 | 7 | 29 | 4.1 |
| 2018-01-01 | 8 | 31 | 3.9 |
| 2018-07-01 | 8 | 47 | 5.9 |
| 2019-01-01 | 11 | 54 | 4.9 |
| 2019-04-04 | 13 | 72 | 5.5 |

Frontend:
| Date | Maintainers | Developers | Developers per maintainer |
| --- | --- | --- | --- |
| 2017-01-01 | 3 | 15 | 5.0 |
| 2017-07-01 | 5 | 14 | 2.8 |
| 2018-01-01 | 6 | 15 | 2.5 |
| 2018-07-01 | 7 | 17 | 2.4 |
| 2019-01-01 | 8 | 30 | 3.8 |
| 2019-04-04 | 7 | 38 | 5.4 |