Add fallback for envrionment variables

[jjcollinge]

If the parameters are not found via flaeg, fallback and check environment variables - this will allow users to override environment variables in isolated environment specific parameters files without having to embed them in the ServiceManifest.xml. This will help avoid having the appinsights key checked into source control and support multiple environments.

[lawrencegripper]

Would this be something we could PR into flaeg rather than implement here?

[serbrech]

I don't think that fits flaeg. Flaeg does 1 thing : parse command line arguments. Handling multiple configuration sources sounds like another layer that could use flaeg.
In fact I think Containous built exactly that : https://github.com/containous/staert
except it doesnt support environment variable... yet :)

Three native sources :
Command line arguments using flæg package
TOML config file using toml package
Key-Value Store using libkv and mapstructure packages
An Interface to add your own sources

I can spike that if you guys agree.

[serbrech]

linking to the request : containous/staert#35

[jjcollinge]

Nice find @serbrech - would be great if we picked up that PR and finished of the work - assuming we're most of the way there.