WIP: Add support for env parsing.

[ghost]

Expand env vars after yaml expansion

Quick example supporting issue #15
Merged against the move_to_bazel branch instead since it seems like that's where you are going with tests, etc.

Context

When dealing with a ci pipeline it is desirable to be able to merge both
static and dynamic configurations.

This includes scenarios where you want to load an application manifest and merge characteristics like build versions from pipeline environment vars.

Solution

Add support to kexpand to an environment expansion in addition to the yaml expansion.
Leveraging an API like https://github.com/a8m/envsubst (or something that accomplishes the same functionality) to provide this behavior with limited modification.

Remaining work

• Test cases
• Confirm if github.com/a8m/envsubst is the api you want to use
• Passthrough unknown vars as literals