Added asset[] parsing to custom_css

The problem which this Commit refers to is described in
https://github.com/stfalcon/TinymceBundle/issues/110

Problem: It was not possible to use the asset[] method inside a
custom_css path for theming of the tiny-mce editor content.

Fix: The tinymceInit() now parses the config for custom_css for themes
through the getAssetsUrl() function therefore replacing asset[] urls
defined in custom_css of a template to working relative urls