Also reorganized the GitHub README.rst front-page.

Sphinx documenation updates, including:
- Extensively re-wrote the "alternatives to cmd2" section as it was many years out of date
- Reworded a few humorous sections in an attempt to be more professional
- Removed reference to command synonyms/aliases which no longer exist
- Added more detail in a couple places
- Minor rewording and bugfixes throughout

Also added a note to cmd2.py about the expected values for doctest only working for Python 2.7.

Also fixed references to the Script files section.

The docs now use the custom HTML theme from Read the Docs so that when they are built locally they look the same as they will at https://cmd2.readthedocs.io/en/latest/

1) Note about needing GNU Readline or equivalent for tab-completion to work
2) Note about behavior occurring when and empty line is entered is different from parent cmd module

- Updated .gitignore for code coverage result dirs from coverage and pytest-cov
- Updated README.rst to include badge for latest docs from readthedocs.io
- Updated setup.py to have updated info for the next time a release is made to PyPI
- Documentation updates:
    - Bumped version in conf.py to 0.7
    - Updated links in index.rst and alternatives.rst