Added way to silence alias/macro creation

[kmvanbrunt]

Added --silent flag to alias/macro create.
Added --with_silent flag to alias/macro list.

[codecov]

Codecov Report

Merging #999 into master will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #999   +/-   ##
=======================================
  Coverage   97.85%   97.86%           
=======================================
  Files          22       22           
  Lines        4571     4583   +12     
=======================================
+ Hits         4473     4485   +12     
  Misses         98       98           

Impacted Files	Coverage Δ
cmd2/cmd2.py	97.24% <100.00%> (+0.01%)	⬆️

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[anselor]

Is this really needed? Can't you just pipe the output to /dev/null in the startup script?

[kmvanbrunt]

@anselor This feature is a convenience when making alias startup scripts. Instead of manually adding > /dev/null to each line, you can just run alias list --with_silent > aliases.txt. That will output all aliases in the syntax alias create --silent name value to the alias file.