[swift] Parse Swift function comment into each arg/return type

[cjbrooks12]

From orchidhq/Orchid #343, parse comment text from a function into each arg. Also, make the return type null if the function returns void.

[RiuHDuo]

I thought this jazzy file may be useful for you.

sourcekitten decodes swift files to json.
Every function's comment is in the key.doc.comment.
I saw jazzy will decode the comment content when rendering these markdown text to html by using regexp

[cjbrooks12]

That file looks like it will be very helpful, thank you!