Pull --writekey From Environment Variable

[elliottpope]

Is your feature request related to a problem? Please describe.

I am trying to use Honeymarker for some Kubernetes automation. It would be easier to source the Honeycomb API key from a Secret injected into the container environment. Since the provided container is built from scratch there is no available Shell to do environment variable expansion

Describe the solution you'd like

The underlying command line option parser allows for specifying environment variable defaults (jessevdk/go-flags#309). We could add env:"HONEYCOMB_API_KEY" to the WriteKey struct tag here to pull from the environment I believe

Describe alternatives you've considered

I don't want to put the API key in plain text in the K8s manifest since this won't allow me to swap out the key value in different environments (i.e. dev, test, production) while keeping the same manifest definition

Additional context

[jharley]

Closed by #76