WIP: Add keywords search mode

This is not ready to be pulled. It's work in progress, but I'd like to get some code review :)

What seems to work:
- I added a new mode, called "keywords"
- The search in the keywords mode seems to work fine as described in issue #98 


Unfinished tasks:
- highligting of the separate keywords does not work (I did not figure that part of the code yet, so it highilights the whole prefix)
- I did not try color highlighting at all
- Remove debugging and clean out the patch
I do not understand the purpose of this part of the hstr_make_selection function. If I don't filter anything here, things seem to work fine.. (you can see my attempt commented out) - can you elaborate?