Support for directory-only negation

[szczeles]

While integrating your library I found one case that is a mismatch with how .gitignore works. It relates to support of directory-only negation - useful thing when you want to discard some spool dir, but keep the directory structure in the version control. The feature is used by Kedro framework in their proejct template:

# ignore everything in the following folders
data/**
# except their sub-folders
!data/**/
# also keep all .gitkeep files
!.gitkeep

and makes it easy to store the structure of data/ subdirs in git:

$ tree -a data/
data/
├── 01_raw
│   └── .gitkeep
├── 02_intermediate
│   └── .gitkeep
├── 03_primary
│   └── .gitkeep
├── 04_features
│   └── .gitkeep
├── 05_model_input
│   └── .gitkeep
├── 06_models
│   └── .gitkeep
└── 08_reporting
    └── .gitkeep

7 directories, 7 files

This PR fixes the directory-only-negation support, together with a small test. You can also verify the behaviour with pure git:

$ cat > .gitignore <<EOF
> data/**
> !data/**/
> !.gitkeep
> !data/01_raw/*
> EOF
$ git init
$ mkdir -p data/01_raw/
$ touch data/01_raw/.gitkeep
$ touch data/01_raw/data.csv
$ mkdir -p data/02_processed/
$ touch data/02_processed/.gitkeep
$ touch data/02_processed/data.csv
$ git add -A
$ git status
On branch master

No commits yet

Changes to be committed:
  (use "git rm --cached <file>..." to unstage)
	new file:   .gitignore
	new file:   data/01_raw/.gitkeep
	new file:   data/01_raw/data.csv
	new file:   data/02_processed/.gitkeep

If you're OK with my modifications, I would be glad if you can release it as a new version of pypi, so I can integrate non-fork version in my project. Thanks :-)

[mherrmann]

Thanks! Merged and published to PyPI as v0.1.1.