Support default argument values

[flwyd]

Vim 8.1.1310 added default values for function arguments, e.g.

function Greet(greeting = 'hello')
  echo a:greeting 'world'
endfunction

vimdoc currently parses this as a two-arg function with parameters greeting and hello: Greet({greeting}, {hello}).

[dbarnett]

Awesome, yeah, we should definitely support this style!