Add backup command

[anf]

• Minor changes to appveyor.yml and codecov.yml
• Create backup command and all its sub-commands
• Minor changes to gitignore.
• Minor changes to how assertions work.
• Added more unit tests.

[codecov-commenter]

Codecov Report

Merging #53 (5309ebd) into master (ecaf5bc) will increase coverage by 11.06%.
The diff coverage is 92.16%.

@@             Coverage Diff             @@
##           master      #53       +/-   ##
===========================================
+ Coverage   68.18%   79.24%   +11.06%     
===========================================
  Files           3        5        +2     
  Lines         264      477      +213     
  Branches       26       53       +27     
===========================================
+ Hits          180      378      +198     
- Misses         65       71        +6     
- Partials       19       28        +9     

Impacted Files	Coverage Δ
WinPath/src/Backup.cs	90.65% <90.65%> (ø)
WinPath/src/Options.cs	100.00% <100.00%> (ø)
WinPath/src/Program.cs	90.83% <100.00%> (+4.96%)	⬆️

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