This commit contains a very large number of trivial changes in order to fix flake8 errors and warnings.  Predominantly these are whitespace changes.

Additionally, the build for Python 3.7 on TravisCI has been tweaked to fail if there are any flake8 errors using the following commandline:
* flake8 . --count --ignore=E252 --max-complexity=31 --max-line-length=127 --show-source --statistics
NOTE: In the future the max cyclomatic complexity should be lowered, but some improvements need to be made first.

One flake8 error is being ignored entirely:
* E252 missing whitespace around parameter equals
    * ignored because it doesn't correctly deal with default argument values after a type hint

A few flake8 errors are being selectively ignored in certain files:
* C901 fuction is too complex
    * ignored in argparse_completer.py because the complex code is an override of argparse complexity
* E302 expected 2 blank lines after ...
    * ignored in all unit test files for convenience
* F401 module imported but unused
    * ignored in cmd2/__init__.py because imports are for convenience of cmd2 developers and backwards compatibility
* F821 undefined name
    * ignored in cmd2 script files which are intended to run only within cmd2 applications via pyscript where "app" and "cmd" are defined

This causes the "invoke tag" command to function in an intuitive fashion as so:
    invoke tag tag_name

Previously when name was an optional argument, the name needed to be set using a flag like so:
    invoke tag -n tag_name

This has the side benefit of now making the recommended usage in the Contributor's Guide correct.

Also:
- The pypi and pypi-test invoke tasks now have the validatetag task as their first prerequisite
- So attempting to publish to pypi without a Git tag will fail

Also:
* Improved Sphinx build options in Makefile and via invoke by adding the following options:
    * -n : nit-picky mode, warn about all missing references
    * -v : increase verbosity (can be repeated)
    * -W : turn warnings into errors
    * -T : show full traceback on exception

Also:
- Made some minor formatting changes in argcomplete_bridge.py to make PyCharm happy
- Fixed typo where "invoke clean" was cleaning up .pytest-cache instead of .pytest_cache