Prepare CLI for additional commands.
Parse commit history based on a set of rules defined for the project.

The rules currently must be defined in a file named `.gitra-rules.yml`,
which has the following format:

```yaml
Story:
  - ^Story (\d+)
Bug:
  - ^Bug[^\s]* (\d+)
```

Usage:
- The major categories go in the first level.
- The second level contains the regexp patterns to match for, without
  the leading/trailing `/` character.
- The second level will be the *numeric* group captured in the patterns.