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.

The alternative pattern I found in CSS.tmLanguage: (#)([0-9a-fA-F]{3}|[0-9a-fA-F]{6})