support colliding CSS filter effects functions

CSS filter effects [0] define several functions (invert, grayscale,
opacity, saturation) that collide with the names of built-in Sass
functions. Overload these functions with a no-op when the filter effects
function signature is discovered. This is compatible with Ruby Sass.

For example, calling `invert` with a color will return an inverted color
as before:

    color: invert(#333) => color: #cccccc

But calling `invert` with a percentage will pass through the function
call unharmed.

    filter: invert(30%) => filter: invert(30%)
    filter: invert(.03) => filter: invert(.03)

Fixes #151.

[0]: http://www.w3.org/TR/filter-effects/