Add support loading config from ENV

[ronin]

This allows to set :key via environmental variables on platforms such as Heroku without the need to rebuild the application.

Here is how to use it:

# config/prod.exs
config :scout_apm,
  key: {:system, "SCOUT_API_KEY"}

[itsderek23]

Thanks @ronin!

We plan on reviewing next week.

/cc @cschneid

[cschneid]

Extended, then merged via #39

Thank you for the code! I moved things around a bit, and took the opportunity to extend this agent to feature-parity with the ruby agent's automatic location of environment variables.