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
check body.classList for search-page when determining to autocheck restrict_sr
Merge pull request #1558 from andytuba/1448

Make restrict_sr checkbox not tick with multireddits