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.

I have added g_source_remove in plugin_cleanup because I use callback notification.