Convert cmd2 to a multi-file Python package

[tleonhardt]

Thie PR converts cmd2 to a multi-file Python package with a __init__.py and source code located in the cmd2 directory.

All unit tests pass (at least on my system) and the Sphinx docs appear to build correctly.

Installation via pip and releasing to PyPI remains to be tested.

This closes #356.

[codecov]

Codecov Report

Merging #360 into master will increase coverage by 0.02%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #360      +/-   ##
==========================================
+ Coverage   89.31%   89.33%   +0.02%     
==========================================
  Files           1        2       +1     
  Lines        1872     1876       +4     
==========================================
+ Hits         1672     1676       +4     
  Misses        200      200

Impacted Files	Coverage Δ
cmd2/cmd2.py	89.32% <100%> (ø)
cmd2/__init__.py	100% <100%> (ø)

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[kotfu]

This looks good. No need to import all the internal stuff by default.

One question, should we import History and HistoryItem by default? I agree with how you've done it here. Most people aren't going to write their own history commands, and if they do, they can easily import the items. If you agree, then I think this is good to merge.

[tleonhardt]

History and HistoryItem are mostly intended for internal use only. Anyone is welcome to use them, but I would view that as uncommon. I'll go ahead and merge.