Support -webkit- prefixes and repeating gradients

First of all, thanks for making this, it's already come in very handy.

When I used it on some old sites I'd written, I noticed it didn't handle `-webkit-` prefixed gradients at all. When I dug into the code I realised it also wouldn't handle `repeating-linear-gradient` either, due to the regular expression used.

This PR fixes up the regex and adds a couple of extra gradient syntaxes to the test page.

Cheers,
Gil


PS. Having written devtools extensions and also struggled with the dev/test cycle, I think the method you used in `test/development.html` to speed up development is genius! I'm going to shamelessly steal it for my own projects. ;)