Adds customizable list of items to look for when searching for project root

[tplunket]

Previous functionality added an optional hunt for project root so that the working directory would be changed to that root before starting ag. This facilitates searching the entire project. However, the list of items was hard coded so changing that list required locally modifying the plugin. This change moves the list of items into a global that can be overridden in the user's .vimrc. This change also adds the tags file as a default item that locates the project root.

[Numkil]

+1 Nice!