add stylus version of noisy uris

Adds a stylus css-preprocessor version for noisy-uris. Instead of individual variables, which require remembering the individual variable naming scheme, created a stylus function, noise(), which takes a type and an alpha value, and returns a url(_noisy-uri_).

noise(type, alpha)
noise(base, 20)
noise(dark, 60) 
noise(light, 100) 

dark() and light() are built-in stylus functions, so there's a bit of internal renaming of type to keep public naming conventions.