Selective exclusion

This update includes exclusion on selected domains. For example, you might want to exclude websites like imgur and reddit where Downworthy's modification of comment text can be confusing (takes a while to figure out why everything is "absolutely commonplace").

I also saw a couple FIXMEs in the `background.js` file; I might have fixed a few by adding [this line](https://github.com/WChargin/downworthy/commit/a9933ffdc3d082194c40d083a8fab93f4d386f3d#diff-8372ed7678044027222f5747e7fafdd1R23).