Use Github's sort order

[jgilchrist]

The existing sorting method uses a couple of heuristics to sort - however, there are cases where the resulting sorting does not match the order in which Github displays the files.

As a result, scrolling down the list can cause the selected file in the tree to jump around rather than going down one-by-one.

This commit removes most of the sorting logic and instead uses the index of the name of the file on the page, guaranteeing the tree matches Github even if they change their ordering logic.

Fixes #187