cmd2.Cmd.cmdloop() now returns self.exit_code which should be an integer

Also:
- Refactored examples to call sys.exit(app.cmdloop()) in their __main__
- Running transcript tests now sets the exit_code accordingly based on success/failure
- Updated CHANGELOG
- Updated README
- Updated Sphinx docs
- Added unit test for case when transcript test fails

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

Specifically:
- PAGER environment variable is not used by default to set cmd2.Cmd.pager
    - This is done to ensure a consistent behavior of cmd2 applications across users by default

Developers are free to set pager and pager_chop in the __init__() for their class derived from cmd2.Cmd differently to ensure whatever behavior they desire.

Also:
- Updated the paged_output.py example to demonstrate using ppaged() with both wrapped and chopped/truncated text

Also:
- ppaged() now uses the pager settable parameter
- By default the pager settable parameter uses the PAGER environment variable if present
- If PAGER environment variable is not present, pager gets set to:
    - "less -SXRF" on POSIX OSes (macOS and Linux)
    - "more" on Windows