Allow directories customizations

This is a simple change.
I've just added the possibility to specify the following paths:
* `PREZTO_DIR`: The prezto directory
* `PREZTORC`: The path for the prezto configuration file

**This allows the user to use custom locations, which is really useful sometimes...**

I've tested the changes, it will work without breaking compatibility. By default, the paths are the current values, so users can just ignore the change if they aren't pro-users and don't want custom locations.

Thanks