Syntax highlighting doesn't fully handle slashy strings / dollar slashy strings

[mparry]

There are a couple of problems with syntax highlighting for strings:

• It doesn't handle interpolations in slashy strings.
• It doesn't handle dollar slashy strings at all.

So, for example, neither of the following strings will currently highlight properly.

/foo $bar ${baz} boz/
$/foo $bar ${baz} b\o\z/$

And in fact, that Markdown render above has the same issues. As far as I can tell, GH's renderer is also using this TM bundle, via https://github.com/github/linguist/tree/master/vendor/grammars!