(rough draft) Highlight parameters of multi-line module, function signatures.

[voussoir]

Hi,

I wanted to improve the syntax so that it would highlight the parameters inside a multi-line module signature. Before these changes, multi-line modules were unhighlighted or wrongly highlighted:

Now they look like this:

HOWEVER, I am quite sure I made some collateral damage in the sense that the original patterns captured the entire module/function definitions, and my patterns are more focused on getting the delimiters to highlight the stuff in the middle. I am really bad at understanding these tmlangs and basically copied some of Python's and then banged my head against the wall until it barely worked. I'm making this pull request to introduce the topic but if you close this and rewrite it yourself that'd be totally great with me.

Thanks very much for your time and effort.