Fix for org-rainbow-tags (12-digit color codes)

[mtekman]

org-rainbow-tags is a package which autogenerates some nice looking color overlays for org-mode tag names.

However, face names use strings instead of symbols (causing htmlize-face-to-fstruct to detect no color at the face at point), and uses 12-digit color codes which Emacs renders fine but CSS does not (hence the fix to htmlize-color-to-rgb).

This PR adds a general fix for quoted face names, and truncates long color hexes into CSS colors.

htmlize.el: htmlize-color-to-rgb, htmlize-face-to-fstruct, htmlize--truncate-hex

Added some small fixes relating org-rainbow-tags which uses strings for tagnames instead of symbols, and 12-digit color codes.

* htmlize-face-to-fstruct: Sets string faces to symbols
* htmlize-color-to-rgb: Update the regex for readability, truncate hex if longer than 9 characters, assuming 8-digit rrggbbaa css colors are fine.
* htmlize--truncate-hex: New function that converts an arbitrary long color hex to a normal 6 digit hex