Mutually exclusive groups

[kmvanbrunt]

• AutoCompleter now handles argparse's mutually exclusive groups. It will not tab complete flag names or positionals for already completed groups. It also will print an error if you try tab completing a flag's value if the flag belongs to a completed group.
• AutoCompleter now uses the passed-in parser's help formatter to generate hint text. This gives help and hint text for an argument consistent formatting.

Fixes #777

[codecov]

Codecov Report

Merging #784 into master will increase coverage by 0.03%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #784      +/-   ##
==========================================
+ Coverage   97.34%   97.38%   +0.03%     
==========================================
  Files          13       13              
  Lines        3469     3474       +5     
==========================================
+ Hits         3377     3383       +6     
+ Misses         92       91       -1

Impacted Files	Coverage Δ
cmd2/cmd2.py	96.77% <ø> (ø)	⬆️
cmd2/argparse_completer.py	100% <100%> (ø)	⬆️
cmd2/utils.py	98.2% <0%> (+0.35%)	⬆️

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[tleonhardt]

I haven't had a chance to manually test any of the changes, but everything here looks OK