Add tests and CI with Github Actions

[gcushen]

• [x] Add flake8 and pytest framework for testing.
• [x] Add at least one test for importing BibTeX with a test bib file.
• [x] Setup CI with Github Actions

[gcushen]

Basic GitHub Actions now working as of 000b1299d44c8e95fc4426a659bae665ddfeaaf8

See workflow at https://github.com/wowchemy/hugo-academic-cli/actions

Future work:

• Formatting (Isort and Black) will not currently provide errors, so requires reading the log, even if workflow is successful. Better solution would be to auto commit formatting improvements in CI or for workflow to fail if there are formatting changes.
• Add code coverage
• Fix 2 format flow warnings:

/home/runner/.local/share/virtualenvs/hugo-academic-cli-jnkQB8Ka/lib/python3.8/site-packages/isort/main.py:914: UserWarning: W0501: The following deprecated CLI flags were used and ignored: --recursive!
  warn(
/home/runner/.local/share/virtualenvs/hugo-academic-cli-jnkQB8Ka/lib/python3.8/site-packages/isort/main.py:918: UserWarning: W0500: Please see the 5.0.0 Upgrade guide: https://pycqa.github.io/isort/docs/upgrade_guides/5.0.0/
  warn(