Attempt to address #114

[S0AndS0]

The result of function_arg Regular Expression now uses labels, however, because we can have a list of comma separated arguments to parse this requires a little bit of list comprehension.

Note: in the future it'd be nice to have @default automatically get assigned when args_groupdicts has both key and value are not None, but at this point I'm happy enough to have Vim 8 syntax supported when building help documentation.

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