New option data and some minor bugfixes

For a project I need to have in Javascript the ids of users mentioned, but not the message with mentions in the form of @[1234], so I added the option data to make the plugin put id_map in the elem.data() (via jQuery).

Also I fixed a little bug when you have multiple @ in the textarea, only the last @ worked as expected.

Finally I fixed another minor bug where a space was always inserted after the selection, even when not needed.