Prefix plugin implementation of gradient

[jossmac]

Declaring background: 'linear-gradient(...)' on an element returns invalid syntax:

background: -webkit-linear-gradient(rgba(0,0,0,0.5), rgba(0,0,0,0)),-moz-linear-gradient(rgba(0,0,0,0.5), rgba(0,0,0,0)),linear-gradient(rgba(0,0,0,0.5), rgba(0,0,0,0));

Likely related to #34

[lesha1201]

I'm going to fix it.