Add parsing options to support GitLab syntax

It does say in the name that this isn't intended for GitLab but besides the obvious `--codeowners` option, how much work for the package would it be to be able to skip section headings within the codeowners file? For this sample GitLab syntax, that means skipping lines 1 and 5

```
[First Section]
some/path/one/a  @one
some/path/one/b  @one

[Second Section]
some/path/two    @two
```
