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

- colors setting now allows Always, Terminal, Never values
- poutput() and perror() honor these new settings

Added support for tab completing application commands in ipython shell
Updated unit tests scripts to set cmd_echo to True to validate command output.

Removed all legacy pystate objects.
Changed default behavior to clear _last_result before each command
Added utility for creating named tuples with default values
Added tests to exercise new changes.

runcmds_plus_hooks can accept multiple commands process the command queue to
 deal with subsequent commands loaded from scripts without requiring a command
 loop. This better supports a one-off batch processing scenario.

Also fixed the insertion order of commands placed in the command queue by load
and _relative_load so that script commands are run in the expected order.

Minor tweak to setup instructions in CONTRIBUTING.md to include pyperclip in
prerequisites.

Also removed default argument for _relative_load and load