mask support

[zslabs]

Hey there! I'm looking to swap out my current postcss, autoprefixer, and cssnano implementation with this lib. One item I ran into is an expected -webkit prefix for mask which didn't work in my testing. Here's a few details:

Playground
can-i-use
MDN

I did see #9 which lead me to think the basic syntax would be supported (outside path and shape), but let me know if I'm off with my assumption there. Thanks as always!