Allowed optional relative paths/environment variables in includes

[jackd]

Love this package. One thing I've been using is relative includes, and this is potentially buggy without doing dirty hacks to prioritize relative paths.

Also added variable/path expansion, because I just like it. Update: this was originally implemented as part of the relative paths reader, but I've abstracted it into an independent function. I can open a separate PR for this if desired.

There's also a slight bug in that use as a decorator didn't return the function as stated in the documention. This is fixed here.

I'm also unsure how to interpret the documentation example with use as a decorator. Is this out of date?