Allow JSON input to `cmioc encode`

[guidanoli]

Input should come from stdin given that it's funny to pass a JSON as a command line argument.
The CLI could decide to read a JSON from stdin if no options are given, like --payload, --destination, etc.

[guidanoli]

What do you think, @edubart?

[edubart]

I think it is a good solution, fallback to stdin in case there is no arguments.