Update README.md.
Update TODO.
Refactor core.py a bit.

Before, for every keyword in the match group core would search through every page.
Now every page from a board is looped over only once per match group.
Funnily enough, this seems to result in a performance loss or around 1s but
I didn't bother to profile and the config file didn't have many entries
with a lot of keywords in them.