Implement helper function to retreive environment variables

[twobiers]

Description of your changes

This changes adds a helper function to retrieve environment variables from the request context.

Fixes #23

I have:

• Read and followed Crossplane's contribution process.
• Added or updated unit tests for my change.

[bobh66]

Thanks @twobiers - I like the idea of a helper to retrieve data from the environment. I think it needs to be flexible enough to handle any data type that it finds or we'll see exceptions when the data is anything other than a string.