Create page if no match is found when using _go

We recently deployed gitit as our corporate wiki at my company, and we love it, but we have a couple complaints about some usability issues from non-technical users. 

The first was that they couldn't figure out how to create new pages. They assumed it worked like wikimedia and you use the 'go' box. But this would go to a existing page if there was a prefix match which was unexpected and undesired behavior because they wanted to create a page that was a prefix of an existing page. This pull request fixes that issue.

The second issue is #396