Fix for data encoded image within cssURL

A typical data encoded image in a `url()` group will start with
something like this:

```
data:image/svg+xml;base64,...
```

The `;` character wrecks havoc on `cssAttrRegion` because it closes the
group. Using `extend` on cssURL prevents the `;` from breaking out into
`cssAttrRegion` match and killing the rest of the syntax highlighting.
Changed cssAttributeSelector to not be transparent

It also inherits the String style, which I think is appropriate since
the syntax for it can be all over the place
Merge pull request #24 from amadeus/fix-cssurl

Fix for data encoded image within cssURL
Merge pull request #25 from amadeus/improve-attribute-selectors

Changed cssAttributeSelector to not be transparent
Adding good support for css noise

I've added a highlight group to ; and : within css declarations
Adding / to cssNoise

The / is used in places for various screen aspect ratios and also font
declarations to separate font size and line height.
Fix for cssDefinition

By adding keepend to cssDefinition we ensure that if the final attribute
in a css definition does not include a ;, it will still be closed by the
cssDefinition's final }.

As per the css spec, the final property does not require a ;, and this
is still technically valid css.