Proper built-in color functions highlighting

[evenfrost]

For functions like hsla, hsl, rgba and rgb Stylus supports the following syntax:

body
  color hsla(#000, .5) // make it semi-transparent
  background rgb(#fff) // fwiw

Would be great if the package could properly highlight such cases as well.

[grassator]

@evenfrost Thanks for the report, should be fixed now