Refactor and changes to support custom user_home.

[kigster]

We only use Dir.home in a couple of places, so it's not a bit deal to allow user_home to be overridden.

This is now possible via the --user-home/-u [DIR] flag.

Utility:
   The following flag helps with Sym installation by hooking to
   your ~/.bashrc. If you are running Sym on an environment without
   user home available, you may need to force set user's home to any existing
   directory using the --user-home flag.

  -B, --bash-support     [file]     append bash completion & utils to a file
                                    such as ~/.bash_profile or ~/.bashrc
  -u, --user-home        [DIR]      Overrides ${HOME} ==> supports AWS Lambda

[fixes #25]

[codecov]

Codecov Report

Merging #30 into master will decrease coverage by 3.0%.
The diff coverage is 92.4%.

@@           Coverage Diff            @@
##           master     #30     +/-   ##
========================================
- Coverage    92.3%   89.3%   -3.1%     
========================================
  Files          49      49             
  Lines        1524    1442     -82     
========================================
- Hits         1408    1288    -120     
- Misses        116     154     +38     

Impacted Files	Coverage Δ
lib/sym/app.rb	63.4% <ø> (-31.3%)	⬇️
lib/sym/app/cli.rb	91.2% <83.3%> (-6.3%)	⬇️
lib/sym.rb	95.3% <100.0%> (ø)
lib/sym/app/cli_slop.rb	98.8% <100.0%> (-1.2%)	⬇️
lib/sym/app/commands/bash_completion.rb	87.8% <100.0%> (+4.8%)	⬆️
lib/sym/constants.rb	100.0% <100.0%> (ø)
lib/sym/errors.rb	95.4% <100.0%> (-0.4%)	⬇️
lib/sym/extensions/with_timeout.rb	66.6% <0.0%> (-33.4%)	⬇️
lib/sym/extensions/with_retry.rb	40.0% <0.0%> (-20.0%)	⬇️
... and 11 more

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