Pipe chaining

[kmvanbrunt]

• Added capability to redirect pipe commands and chain them together. This involved changing Statement.pipe_to from a list to a string.
• Fixed parsing issue in case where output redirection (e.g. > file) appears before a pipe. In that case, the pipe was given precedence even though it appeared later in the command.
• Preserving originally typed quotes of Statement.output_to for use in Statement.post_command()

[codecov]

Codecov Report

Merging #676 into master will increase coverage by 0.16%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #676      +/-   ##
==========================================
+ Coverage    95.1%   95.27%   +0.16%     
==========================================
  Files          11       11              
  Lines        3168     3175       +7     
==========================================
+ Hits         3013     3025      +12     
+ Misses        155      150       -5

Impacted Files	Coverage Δ
cmd2/parsing.py	100% <100%> (ø)	⬆️
cmd2/cmd2.py	95.55% <100%> (+0.2%)	⬆️
cmd2/argparse_completer.py	89.7% <100%> (-0.02%)	⬇️
cmd2/utils.py	97.59% <100%> (+0.55%)	⬆️

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[tleonhardt]

This PR contains a really nice set of enhancements and bug fixes that make cmd2 applications behave significantly closer to how a "normal" shell behaves. Nice work!

[tleonhardt]

@kmvanbrunt @kotfu Do either of you understand the AppVeyor Windows CI failures? From what I can see they are not actually related to this PR, but are some systematic problem with AppVeyor at the moment and/or how we have it configured. However our AppVeyor configuration hasn't changed recently in 7 months.

[tleonhardt]

I pushed a trivial update to to README.md on the master branch and verified that the AppVeyor issue is completely unrelated to anything on this branch.