Since there is no established format for representing collections within env variables, to enable their support, the feature with a custom parser attribute added.
Add helpers for parsing env variables into collections
Improve rustdoc for `env_utils` functions
Improve `parse_env` example
Fixes#2
I simply forgot to use the `deserialize_with` attribute for the env
deserialization. The previous code was somewhat weirdly coded in that
we would always deserialize `Option<T>` (as it's a partial type) and
the "env variable not present" info would travel through the
deserializer to the `Option<T> as Deserialize` impl. Now it's more
straight forward.
