Add new stage:draft key/value pair allowing to exclude draft PRs

[sbesson]

Fixes #262

This is a simple expansion of the key/value filter infrastructure that allow to test the draft attribute of a Pull Request and exclude it from the merge candidates. The default org set of filters is modified to exclude draft PRs by default.

For testing, first install this branch e.g. in a virtual environment

source venv/bin/activate
pip install -e .

Then run the command against a repository against which Pull Requests in a draft stage are opened e.g. https://github.com/ome/bioformats/pulls. The default set of filters should now exclude these PRs from the merge candidates and indicate the reason in the log output:

scc merge info --master # should filter out draft PRs
scc merge info --master -Dall # should include all PRs including draft PRs

Proposed tag: 0.19.0 given the breaking change

[will-moore]

I've not tested this, but it seems like a good feature (expected behaviour). 👍

[sbesson]

Agreed, though now I feel guilty we never went 1.0 ;)

Happy to call the next version 1.0.0 if it's preferable.