Add Env parser parameter.

When passing values via the environment, contrary to the other methods provided (json file, direct add/set, etc), all values are strings. While the technical reasons for this are clear, it would be good to have a possibility to convert these values into the type they are intended to be.

The solution I propose is a very simple one: Add an optional parameter to the env loader, which is used as a parser. If not present, or the parser throws an error, the original value in it's string type is used.