Added rule for preprocessor directives in Swift

Preprocessor directives always take up the whole line in Swift so the rule's pretty simple.

I used this to highlight the code here: http://giorgiocalderolla.com/cross-platform-swift.html
`^` anchors have known weird behavior.  It's probably better to just push a state when you see a newline, mixin comments / whitespace, check for the `#`, and pop with an empty regex.