Fix processFullPosts to use element parameter

The XPath expression was always evaluated from the beginning of the
document no matter what the argument passed to processFullPosts was.  It
will now only evaluate the node passed in, which seems to be the
intended behavior when looking at existing calls to processFullPosts.
Added missing heading tag in options.html
Do not install elder scroll on single threads

Oops.
Prevent loading dupe threads (elder scroll)
Merge pull request #26 from awodecki/master

ChromeShack.processFullPosts fix & "elder scroll" script update