Custom API plugin

This plugin makes a post request to a specified custom URL, with optional support for HTTP basic authentication. It also supports sending arbitrary URL-encoded data along with the IPA.

Usage:
`ipa distribute:api --host http://api.someapp.com -u username --path "/version.json" --data version=1.0&hello=world`

This is essentially the Testflight plugin extrapolated to work with any API.

The use case arose from a need within our firm. Our enterprise client apps are distributed from a simple Rails app. This simple addition to Shenzhen allows us to build and deploy through our custom app from the command line.