Add the ability to set an environement vatiable for Kentik credentials

[mkrygeri]

You should be able to set the env variables and these will be added to the command line options without being exposed to the process information outputs export KENTIK_API_TOKEN=blah export KENTIK_API_EMAIL=blah@blah.com Has a hard time building with the previous "ring" crate, so updated this one.

Will complain if both CLI options and env is set.

[ghost]

Thank you! This isn't quite the right approach, but I've implemented support for env vars in https://github.com/kentik/kprobe/commit/ad3c15f5610a3cc6baa57176d51fae1bfa5b4b0c