cmd2-specific input() function

[kmvanbrunt]

Closes #778

[codecov]

Codecov Report

Merging #810 into master will decrease coverage by 0.07%.
The diff coverage is 96.22%.

@@            Coverage Diff             @@
##           master     #810      +/-   ##
==========================================
- Coverage   97.33%   97.26%   -0.08%     
==========================================
  Files          14       14              
  Lines        3493     3504      +11     
==========================================
+ Hits         3400     3408       +8     
- Misses         93       96       +3

Impacted Files	Coverage Δ
cmd2/cmd2.py	96.54% <100%> (+0.01%)	⬆️
cmd2/utils.py	97.17% <66.66%> (-1.05%)	⬇️

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