A Few Improvements to SCSS Mode

This adds detection for variables in SCSS mode (I think this previously existed, but was removed without comment in @7e3b127), along with improved detection of functions.  

I also have a few questions, if anyone's available to answer them -

1. Should function arguments be classified as functions? This leads to the parentheses and any unclassified arguments to be colored the same as the function, which strikes me as a bit odd. 

2. Would it be appropriate to create a CSS-specific form of NUMBER_MODE that includes units? Right now, a value like `10px` would classify the `10` as a number and leave `px` alone (this is true for both CSS and SCSS). To me, it seems reasonable that units like `px`, `em` and `deg` should be classified with the number.