Allow the caller to control whether read_env will recurse.

[gthank]

The underlying library already exposes a flag to control this behavior,
so I'm merely adding a passthrough for that flag.

I think this flag should be exposed for the same reason it's exposed in the underlying library: it's very useful. For instance, I don't want a web service process potentially recursing up the file system out of its little "jail" (or at least trying to); it's preferable to me that it just throw an Exception. Since that's already something provided by the underlying library, it seems like a good idea to expose it here. I made the default True so the change should be backward compatible.

[sloria]

Makes sense to me. Thanks!