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
Yup, they can. So I tried changing the regex to `rule /^\s*#.*/, Comment::Preproc` but for some reason it doesn't work :) It works in irb but not in the gem :/