environment variables are replaced in conf file

- environment variables of the form ${VARIABLE} are replaced
  with their contents before loading the configuration file -
  this is to allow keys, secrets, and passwords that should
  not be checked into source control to be referenced in
  the configuration file
fixing typo in README.md
Merge pull request #8 from adamfeuer/config-gets-keys-from-env-vars

environment variables are replaced in conf file