Support ! in .capstanignore

[miha-plesko]

See this comment #91 (comment) where @aitorhh requested such feature.

It wouldn't be too much of a trouble to at least support very basic functionality (without any wildcards):

# explicitly include specific file (must not contain *)
!/path/to/file.txt

# explicitly include whole directory and its content (must not contain *)
!/path/to/directory

@aitorhh would this be sufficient for your usecase?

[aitorhh]

Yes, I think allows the users to be more selective with the files that want to include. And allows a broader set of project structures to be used :)

Thanks!