Improve HEX_REGEXP

As Sublime syntax highlighter does, it matches the color only if there is 3 or 6 characters (after the #), and will not render a preview of the first 3 characters. Otherwise it may be confusing and beginners could be tempted to try 4 or 5 characters values.

![capture](https://f.cloud.github.com/assets/5932082/2441587/4d5e8124-ae16-11e3-93b4-bdc36cc8e90c.PNG)