Optionally remember window configurations when switching projects.

This pull request allows for a project's most recent window configuration to be restored as a project switching action. More specifically, you can now set `projectile-switch-project-action` to `projectile-restore-window-configuration', resulting in that project's latest window configuration to replace the contents of the current frame. If the target project has no recent window configuration, `projectile-switch-project-action-alternative` is called.

Note that there's no caching of window configurations, since they have no read representation as lisp objects, making them difficult to serialize.