formatters: Remove Hoedown formatter, split it off to a pluging. Its availability was spotty depending on the platform (because `misaka` doesn't have binaries available for even Windows, for instance). So it was split off to a plugin that the user can install if needed.

sudo: false
language: python
python:
  - "3.4"
  - "3.5"
  - "3.6"
install:
  - "python setup.py version -f 2.0"
  - "python setup.py sdist --formats=zip"
  - "pip install dist/PieCrust-2.0.zip"
script:
  - "py.test"