Ipypublish

Changes include:

- implement `HTML2JSONParser` and `pytest-regressions` for testing HTML documents

Rather then searching for specific bits of text, we convert the document to a json object, stripping out sections irrelevant for testing (like the header, footer and scripts that can change between versions). and test with the `data_regression` fixture

- Add pre_commit configuration
- Update .travis.yml to test python 3.7 rather than 3.4 (which is deprecated)
- Update RTD configuration
- version bump

- remove `sphinx.ext.imgconverter` from sphinx auto-builds
- add additional known sphinx roles

- Only require backport dependencies for python version older than their implementation
- use `ordered-set`, instead of `oset` dependency, since it is better maintained

- Formatting of the execution_count is now inserted by: `ipysphinx_input_prompt.format(count=execution_count)`
- use "Code Cell Output" as placeholder for output image caption