Filters and actions additions

A few additions --

* Adding a sanitize function, as originally it doesn't look like the CSS was hitting any sanitizing code to pull out any naughty maliciousness or validation before saving to the db.
* Adding an action after write_css runs to let other plugins catch on and minify the css themselves if desired, or push out to a CDN, or share among load balanced servers.
* Adding filters or actions to let other plugins change the datastore from an option to a theme_mod or CPT if desired.