teach rainbow-mode about zenburn colors

Teach Rainbow mode to also add font-lock keywords for Zenburn color
identifiers (symbols but actually all text occurances).

* zenburn-colors-alist: hold list of zenburn colors and their values
* zenburn-with-color-variables: macro to let bind these color symbols
* zenburn-add-font-lock-keywords: whether to enable add zenburn
  keywords in all elisp buffers, by default only in zenburn-theme.el
* zenburn-turn-on-rainbow-mode: turn on rainbow-mode if it is
  available.

The activation of rainbow-mode has been improved.  Users now only get
a message about a potentially unsave eval form if they have neigher
loaded nor autoloaded zenburn-theme before visiting the file.