Add option 'auto_hide_read_on_mobile' support.

# Summary:

Allows to hide read item when closing fullscreen view.
This makes reading RSS on mobile easier and has been sucessfully tested on smartphone Galaxy Nexus and on tablet Nexus 7 & TF101 with Firefox for Android and default Android web browser.

# Technically:

Read items are hidden when closing fullscreen view only if:

* option `auto_hide_read_on_mobile` is enabled
* item's state is unread (ie. when item does not have class `unread`).

Default value for `auto_hide_read_on_mobile` is disabled.