add subcommand aliases

[alexflint]

With this PR it is possible to define aliases for subcommands like this:

var args struct {
    Remove *struct{} `arg:"subcommand:remove|rm|r" help:"remove something from somewhere"`
}

You can then write ./myprogram remove or ./myprogram rm or ./myprogram r

They show up in the help output like this:

Usage: example <command> [<args>]

Options:
  --help, -h             display this help and exit

Commands:
  remove, rm, r          remove something from somewhere

Originall I wanted to use tag syntax subcommand:remove,rm,r but I couldn't because the commas are already defined to be a separator at a higher level so it would create trouble to write something like subcommand:blah,help:something. I decided to go with pipes instead.

Closes #230

[codecov-commenter]

Codecov Report

All modified lines are covered by tests ✅

Comparison is base (5ec29ce) 99.19% compared to head (e7a4f77) 99.17%.

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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #231      +/-   ##
==========================================
- Coverage   99.19%   99.17%   -0.02%     
==========================================
  Files           5        5              
  Lines         745      731      -14     
==========================================
- Hits          739      725      -14     
  Misses          3        3              
  Partials        3        3              

Files	Coverage Δ
parse.go	99.52% <100.00%> (+<0.01%)	⬆️
subcommand.go	100.00% <100.00%> (ø)
usage.go	97.68% <100.00%> (-0.28%)	⬇️

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