fix highlighting of linear and radial gradients

Fixes #15 by adding `linear-gradient`, `radial-gradient`, `repeating-linear-gradient` and `repeating-radial-gradient` to the property-value list. 

Also moves `repeat` down the list so `repeat-x` and `repeat-y`, as well as the repeating gradients are recognised first.

Also moves `keyword.control.html.elements` and `keyword.control.html.custom.elements` to much later in the file, because their regexes interfere with property-values that contain a `-`.

Which coincidentally also fixes #14 .

Before
![before](https://cloud.githubusercontent.com/assets/2543659/5191789/b755eb88-74f1-11e4-8f47-cdda7c72ec0e.png)

After
![after](https://cloud.githubusercontent.com/assets/2543659/5191790/b7574186-74f1-11e4-90de-a29bb2833cb0.png)