Add a throw (mark as read & open next/previous) shortcut.

This patch add two new shortcuts that allow to browse the entries quicker:
```t``` (throw) to mark the current entry as read and open the next one.
```shift+t``` to mark the current entry as read and open the previous one.

This functionnality is inspired by Google Reader where pressing space does the same thing.

I choosed not to allow those shortcuts to switch an entry back to the unread state, since think it does not makes any sense to "mark as unread and open the next/previous one".