Add from_read and from_read_iter.

[KevinMGranger]

This will allow reading env vars from any arbitrary io::Read.

Use case: I'm working on a tool to translate a dhall file into a dotenv file, and a tool to load dotenv files into the fish shell.
I'd like to run dhallenv file.dhall | fishdotenv, which requires reading a dotenv formatted stream from stdin.
(Yes, I could use from_filename_iter("/dev/stdin"), but that's a bit non-obvious, and feels a little hacky.)

I considered exposing Iter directly, but that seems like an implementation detail.

I didn't add additional unit tests because it doesn't really add any new code paths. If that's not okay, I'll happily add some.