Custom parser

[kmvanbrunt]

Added capability to override the argument parser class used by cmd2 built-in commands
Closes #770

[codecov]

Codecov Report

Merging #812 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #812      +/-   ##
==========================================
+ Coverage   97.25%   97.26%   +<.01%     
==========================================
  Files          14       14              
  Lines        3503     3506       +3     
==========================================
+ Hits         3407     3410       +3     
  Misses         96       96

Impacted Files	Coverage Δ
cmd2/cmd2.py	96.54% <100%> (ø)	⬆️
cmd2/argparse_custom.py	94.23% <100%> (+0.05%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 8c00d34...0d6e9cb. Read the comment docs.

[kmvanbrunt]

Does anyone think cmd2_parser_module should be renamed to cmd2_parser_module_name since it needs to be a string?

[tleonhardt]

Looks good. Thanks for adding an example and unit test