Automark: first release

Simple plugin that automatically highlights current word (no hotkeys or so on):

![Automark](http://i.imgur.com/q6YyuN4.png)

Now I should touch 16 files, right? (remembered that magic number)
> I hope that checking with DOC_VALID eliminates all cases of possible segfault. Not only plugin unloading may lead to crash but during callback user may simply destroy that document.

Yeah, `DOC_VALID()` should be good here.

> I have added g_source_remove in plugin_cleanup because I use callback notification.

OK.  Note however that you could replace your destroy notification by simply setting `source_id=0` inside the timeout callback.  Either way it's fine, just wanted to mention it.