Add arguments group

[SebastiaanPasterkamp]

Argument groups allow for nesting structs of logically related arguments. Although similar to sub commands an argument group does not require a trigger keyword (the command) and is compatible with both nested structs as well as struct pointers.

Resolves #179 with test coverage proving environment-only arguments are populated properly, and help messages now properly displays the help line for the (environment only) argument.

[codecov-commenter]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.02 :tada:

Comparison is base (e25b470) 99.72% compared to head (36d760a) 99.74%.

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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #192      +/-   ##
==========================================
+ Coverage   99.72%   99.74%   +0.02%     
==========================================
  Files           5        5              
  Lines         716      775      +59     
==========================================
+ Hits          714      773      +59     
  Misses          1        1              
  Partials        1        1              

Impacted Files	Coverage Δ
parse.go	99.55% <100.00%> (+0.04%)	:arrow_up:
usage.go	100.00% <100.00%> (ø)

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[kaedwen]

Hey @alexflint, I know time is short :-) but this is really a nice feature I love to see in go-arg. If I can help in any case to speed this up just ping me.

[SebastiaanPasterkamp]

@alexflint There were too many confusing merge conflicts with my old version, so I've restarted the PR from just after v1.4.3. The changes and commits have also be consolidated to be more concise, so hopefully the changes are easier to review as well.

[hhromic]

This is a very nice feature! Thanks for opening this PR for it. Another similar feature is to implement argument groups with mutually exclusive arguments. I think once this PR here is merged, I can give it a try to implement the above idea on top of the work from this PR.

[SebastiaanPasterkamp]

@alexflint Are you still interested in this PR?

[alexflint]

Sorry it's been so long without response on this PR! After considering this I think this PR adds too much complexity to the code, relative to the feature it adds.