Add additional arg for stdin to embedded commands

[mpeck]

Currently embedded commands may receive the output from previous commands via req.cog_env. This isn't particularly obvious and we also need to use cog_env for other, more envy things (see #618). To make it more clear embedded commands handle_message function will receive an additional argument that contains the decoded json from the previous command.

def handle_message(req, args, stdin) do
...
end

parent #399

[vanstee]

I was actually wrong about this the command request (passed into handle_message as req) contains all the env vars that we pass to relay, so I think reading previous output from req.cog_env and env vars from req is fine for now.