CombineStorage source

I just added a new source actor:

CombineStorage

This source works similar to the CombineVariables source, as it allows you to combine string representations of storage items ("%{name}") and variables ("@{name}") into a single string.

Furthermore, I added the ExpandVariables and ExpandStorage transformers which expand the variables and/or storage items (using the string representations) in the strings passing through.