scripts: get_maintainer: add count function

For looking at statistics over time, it is helpful to be able to
quickly query the number of unique maintainers, collaborators,
areas, and the number of orphaned areas (without a maintainer).

This change adds the `count` subcommand to
`scripts/get_maintainer.py`.

```
% python3 ./scripts/get_maintainer.py count -h
usage: get_maintainer.py count [-h] [-a] [-c] [-n] [-o]

optional arguments:
  -h, --help            show this help message and exit
  -a, --count-areas     Count the number of areas
  -c, --count-collaborators
                        Count the number of unique collaborators
  -n, --count-maintainers
                        Count the number of unique maintainers
  -o, --count-orphaned  Count the number of orphaned areas

% python3 ./scripts/get_maintainer.py count
areas:          113
maintainers:    49
collaborators:  81
orphaned:       21

% python3 ./scripts/get_maintainer.py count -o
orphaned:       21
```

Signed-off-by: Christopher Friedt <chrisfriedt@gmail.com>