Category Labels (RFC)

Needs comments / suggestions on how to handle regex labels for commitlog.

Currently the categorization works in the following way.

Eg: a catg regex like so would produce the labels
```sh
$ commitlog g --categories="feat:,fix:"
```

```md
## feat:
## fix:
```

Though this doesn't make sense when you have really complex regex's handling the categorisation.

So, there needs to be a way to create labels for these category regexes

One possible way is to change the way the flags are parsed and have more than one instance of the flag.
```sh
$ commitlog --category="Features%%feat:" --category="Fixes%%fix:"
```

Considering the chances of `%%` being together in a regex being very rare and also since the logic would only take the first separation to get the `LABEL` everything else would be compiled back as regex.

Start by reviewing the current category parsing behind the --categories flag and compare it with the proposed repeated --category form. Define how labels should be separated from complex regexes, then verify that category headings are generated with the requested labels for both simple and complex patterns.

索引モデルが issue の本文から書いたものです。