Default sort key

[kmvanbrunt]

Fixes #711

• Renamed matches_sort_key to default_sort_key. This value determines the default sort ordering of string results like alias, command, category, macro, settable, and shortcut names. Unsorted tab-completion results also are sorted with this key. Its default value (ALPHABETICAL_SORT_KEY) performs a case-insensitive alphabetical sort, but it can be changed to a natural sort by setting the value to NATURAL_SORT_KEY.

• Moved the sorting of shortcuts into StatementParser which allows users to pass in a dictionary instead of a tuple for shortcuts

[codecov]

Codecov Report

Merging #721 into master will decrease coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #721      +/-   ##
==========================================
- Coverage   97.15%   97.15%   -0.01%     
==========================================
  Files          13       13              
  Lines        3377     3373       -4     
==========================================
- Hits         3281     3277       -4     
  Misses         96       96

Impacted Files	Coverage Δ
cmd2/argparse_custom.py	93.79% <ø> (ø)	⬆️
cmd2/parsing.py	100% <100%> (ø)	⬆️
cmd2/cmd2.py	96.45% <100%> (-0.01%)	⬇️
cmd2/argparse_completer.py	100% <100%> (ø)	⬆️

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