Add ability to create directories in the editor

Created by: binford2k

Simply type a name with a / directory separator and new directories will be created. This does not do the fancy UI work that github.com does, but it will get the job done.

I could not find tests for file creation, so I didn't add a test for this slight behaviour modification. I did test directory traversals though, using both absolute paths like /tmp/foo.txt and relative paths like ../../foo.txt. Neither case escaped the repository, though attempting to traverse with a relative path resulted in a 500 error that did not affect application stability upon reload.