Add utility function to check folder is a projectile folder or not.

# Add a function named `projectile-project-p`. 

To judge current folder is a projectile-satisfied folder or not.
This function can be used by you in future or in emacs configuration.

## For an illustration:

recentf can do a projectile-scope recentf if it was in a projectile folder( invoke `projectile-recentf`) and do origin recentf when not in projectile-folder.

User do not need to remember more keys. just do a smart-recentf. 
functions such as grep, open, multioccur... all can do in this way.