Refactor default filters logic into get_default_filters()

-D option defines the default set of filters. The default set of included PRs
is read from the "mode" key of the include filter set. Default include and
exclude filters are also defined in get_default_filters().
find_candidates() is modified to use the new filters definition.
Modify test_filters to account for new filter defaults

Use multiple parametrize fixtures to test all combinations of filters
Define list of defaults
Add logic to handle -I mode:MODE filter and add unit tests
Correctly handle none include mode
Include all whitelisting options as part of the user include filter

- Define "org" and "all" user special values
- Refactor is_whitelisted() method to handle a user list
- Improve the _log_filters() logic
Fix filter command descriptions
Factorize setup_org() and setup_repo() in GH repo unit tests
Add unit tests for is_whitelisted logic
Fix integration test in agreement with new filters
Fix typos
Use tuple rather than list
Prepend special org and all user filters using hash symbol
Fix include comment integration test
Fix filter key/value pair parsing to allow leading hash symbols
Merge pull request #145 from sbesson/default_filters

Refactor default filters and whitelisting logic