JSON output for commands

[philnash]

Making JSON output an option for twilio-run commands, should fix #62.

This is WIP so far as I left some questions on the issue.

Questions repeated below:

Should the start and new commands have the --output-format flag as an option at all?

Should those two have another flag/option which silences their output so that other programs can use them without them making a mess over the terminal (e.g. see the test output of create-twilio-function)?

Or should I just leave the start and new commands alone for now?

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