Customize starting directory for utop

[benbellick]

Hi! I opened up #471 . I use utop-mode but wanted to automatically load in my .ocamlinit file at the root level of my project without having to pass it in explicitly using the -init flag. My solution is to enable a customization to start utop from the project root dir.

Now when you set the variable utop-starting-directory (possibly via customize) to 'project-root, project.el is used to find the current buffer's project root. Then utop is called from that directory. If the project cannot be found, an error is thrown.