Make restrict_sr checkbox not tick with multireddit's

Changed line 80 and made + an accepted character in the regex. This means that the Restrict Subreddit checkbox will not be ticked when linking directly to a search using multireddit's.

EG: http://www.reddit.com/r/networking+sysadmin/search?q=UDP+joke
Hmm.. now that you've pointed out this issue, we should probably make it even more general/resilient.

how about replacing this regexp test entirely with something like `body.classList.has('search-page')`?