Add ability to create directories in the editor

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.