Add option to read config var values from stdin

[ThomasWeiser]

Copying from heroku/legacy-cli#383:

Add an option to the Heroku client to reade the value of a config var from stdin.

Something like:
heroku config:add REDIS_URL —prompt —app dark-river-123

By adding an option like "--prompt" or "--stdin" we can solve two problems:

• Ability to easily read secrets from a file using shell redirection
• Ability to paste secrets without showing them on the screen or getting in the shell history
  The heroku client should detect whether standard input is a TTY, and optionally prompt for the secret value, possibly with echo disabled.