feat(templating): introduce postinstall hook

[dkundel]

This feature allows templates to have postinstall hooks that can run account setup for users. It's a sandboxed script that will allow scripts to access Twilio but not other parts of the system.

This is still very much a WIP. Before we can ship this, the following have to be done:

• add new flags to new command that support Account SID and Auth Token
• add documentation to twilio-labs/function-templates
• read env variables from local env file
• command to re-run setup
• tbd

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