Added scrolling threads with up and down keys

For 8chan X. Redone so it's in js folder.
@KillerWhale321
there is a lot of parent()s and a guesswork. if you are searching for a top thread diff, you can do the following: ele.parents('div[id^="thread_"]'). this way, you are sure, that you actually selected a thread over the element under your mouse.