Feature: enable/disable permalinks for RSS

The option to see the shortlinks or permalinks has been added to the configuration panel. It is a simple checkbox
This option is disabled by default (meaning that shortlinks are the default)
Updated writeConfig() to save this option
Also fixed a slight typo in config.html.
Because inline-styling should be removed, both because of https://github.com/shaarli/Shaarli/issues/29 and because *styling* should go into *style* sheets.