Add a `—config` option to the CLI tool

[Nanda H Krishna]

As discussed in #8, we should add a --config option in case people want to use StegCloak in languages other than Node - the current interactive nature of the CLI tool is a problem in those cases.

[Maximilian Burszley]

I'd also like to propose using environment variables for dynamic use-cases. In cli tools, there's usually a preference of cli > env vars > config file.

Something like STEGCLOAK_REVEAL_$OPTION as far as naming convention.