Rain variables

Hi all!

I wish to add the ability to define where to store all writable files. I set up two variables for RainTPL. This way, it will be possible to install/setup Shaarli in two separate directory : one with write access, one with read only access.

I think it would be neat for package maintainer and, maybe, to have a multi-user deployment.

Would you like to check this, at least, the following ways:
- Is the idea valuable?
- Is there any error in it?
- Do you like my implementation?

Thanks in advance.