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
We can sympathize!  It'd be great if you could drop a comment here when you get back to this, so we get a notification.