when developers read the code in cmd2.py.

Lazy loading AutoCompleter in cmd2 instance methods to allow argparse_completer.py to import
cmd2.Cmd class. This Architecture makes more sense because AutoCompleter depends on cmd2.Cmd.

Also:
- Fixed an example in README which was using print() instead of self.poutput()

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

These classes are subtly different, particularly in terms of their truthiness.

CmdResult
- attributes:  out, err, war
- truthy:  if err is falsy

CommandResult
- attributes:  stdout, stderr, data
- truthy:  if err is falsy AND data is not None

So CmdResult was oriented to provide essentially info, error, and warning messages to the user (typically as stirngs), whereas CommandResult is geared towards providing info and error messages to the user as strings in addition to data to the user in a command-specific format which is arbitrary other than it should never be None if the command succeeds.