New page button now respects current directory

Currently, using the new page when viewing a file within a sub-directory causes the new page to be created in the root. This changes this behavior to instead make the page in the same sub-directory as the currently opened page.

I considered adding the current sub-directory as path information on the input box but because of the way the user's input is encoded it would not get interpreted correctly. This opens the question of whether the new page dialog should allow user's to specify a sub-directory.

Because this pull request is a change in current functionality, I'm looking for feedback on the change and a discussion on the best course of action for Gollum.