Feature/book spoilers

- Modified regex to only match what it needs to, i.e. the full spoilers tag and the level of spoilers within that tag
- Pull out spoilers level from matches and compare it to the requested book using the `check_spoilers` method
- Allow search to happen if the level matched in the post title is greater than or equal to the requested book level

Can't test this without the data, but I did manual testing of matching titles with the new regex and comparisons using `check_spoilers`.

One thing to note: `check_spoilers` assumes that a thread tagged as having ADWD spoilers is the same as one tagged as spoilers all. Let me know if you don't want that to be the intended behavior and I can fix it.