Add option to read config var values from stdin

[ghost]

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.

[ddollar]

We could make this default behavior if you dont include the =value part.

[ghost]

That does seem a bit cleaner

[tmaher]

Yeah, do that.

[geemus]

Thanks for the input everybody. It's on my list (just not quite at the top presently). Happy to help out if anybody wants to take a shot at a pull request though.