Finish up spoilers plugin and make sure what we're doing is accessible

- [ ] Figure out how to handle adjacent marks.
- [ ] Make text not bleed through selection (see below)
- [ ] Make sure what is done to avoid bleed works with screenreaders
- [ ] Make sure text is togglable with keyboard

Text bleeding through:

Adjacent marks not merging

Current thinking: if one character from the previous mark is selected and spoilers is toggled, merge; if they are separate selections keep separate.