New feature in project.el: Remembering the previously used projects

[Dmitry Gutov]

Hi all,

Thanks some good work by Simen, project commands (project-find-file, 
etc) now prompt you to choose among known projects, if currently outside 
of all project directories.

It's now on master.

There's also a new command, project-switch-project.

Please let us know how you like the change, and if you see any new problems.