Add parse utility for pkg-config.

A utility to parse out information from pkg-config into a dictionary consumable by python.

```python
config = pkgconfig.parse('gtk+-3.0')

assert ('GSEAL_ENABLE', '') in config['define_macros']
assert '/usr/include/gtk-3.0' in config['include_dirs']
assert '/usr/lib64' in config['library_dirs']
assert 'gtk-3' in config['libraries']
```