add ClickSubcommandTableDirective

[golobor]

Hi @stephenfin !

Thank you for developing sphinx-click, it proved to be extremely useful! One bit of functionality that seems to be missing from sphinx-click is the ability to create summary tables of all subcommands of a click group (e.g., a manually generated example).

In this pull request, I propose a simple directive that generates a table of all subcommands together with their .short_help. You can find an example of a table generated with this directive here.

Please, let me know if you think that this Directive fits into sphinx-click. I would be very happy to implement any requested changes and fixes.

Thank you!

[codecov-io]

Codecov Report

Merging #28 into master will decrease coverage by 18.86%. The diff coverage is 9.52%.

@@             Coverage Diff             @@
##           master      #28       +/-   ##
===========================================
- Coverage   72.22%   53.35%   -18.87%     
===========================================
  Files           1        1               
  Lines         198      283       +85     
  Branches       51       67       +16     
===========================================
+ Hits          143      151        +8     
- Misses         47      124       +77     
  Partials        8        8

Impacted Files	Coverage Δ
sphinx_click/ext.py	53.35% <9.52%> (-18.87%)	:arrow_down:

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[golobor]

bump :)

[stephenfin]

Really sorry about the delay :see_no_evil: This is a great start. In addition to the few comments I've included, I'd also like to see this new directive documented somewhere. Once that's done, I think we're golden