Allow plugins to access specific secret environment variables

[ehmicky]

From #68, we have the following proposed syntax:

plugins:
  pluginOne:
    # secrets by key
    secrets: # accessible by this
      - MY_SECRET_KEY

This means we are passing this environment variable to the plugins, correct? This would require plugins to be run in child processes (see #168).

What about the other environment variables? How do we know which one should be passed or not to the plugin child process?

[ehmicky]

Merged to #842.