Webhook Server

[awm33]

In order to support singer-io/getting-started#50 a webserver must accept HTTP requests and pass them on as WEBHOOK messages.

Proposal for how that could work:

• A singer-runner webhook-server command would start an HTTP server.
• POST /webhooks/:pipe-name/:token
  • pipe-name maps to a specific pipe instance ex mailchimp. When a request is successful, a WEBHOOK message is placed on that pipe.
  • token maps to a single token passed in the config or environment. Perhaps a function or class could be passed in to allow for more complex auth.
• run_tap would need to be changed to allow for input pipes.