Add file blacklist support

[simonmeggle]

Hi Thomas, thanks for this helpful package. I have added a blacklist support to specify exclude patterns for files I do not want to have in it, but have to be still present in the folders. The blacklist is given to dist() as an optional parameter.

Example:

blacklist = [
    'local/lib',
    'test_robotmk_plugin.py',
    'agents/plugins/robotmk.py',
    '__pycache__',
]

dist({
    'author': 'Simon Meggle, https://www.simon-meggle.de',
    'description': pkg_desc,
    'download_url': 'https://www.robotmk.org',
    'name': 'robotmk',
    'title': 'RobotMK | Robot Framework End2End Test Integration',
    'version': tag.replace('v',''),
    'version.min_required': '1.6',
}, blacklist=blacklist)