Implement a `meteor debug` command.

Summary:
The `node-inspector` NPM package was added to the dev_bundle by this
recent commit: 64a624ae5c36a30c10e20216400f7e751d4963b2

Task: https://app.asana.com/0/15750483766338/16241466809965

Test Plan:
Add `debugger` statements to server code, run `meteor debug`, visit the
node-inspector URL in a browser, continue the application, and verify that
the debugger stops at the `debugger` statements that were set.

Reviewers: nim, slava, emily, avital, dgreenspan

CC: sashko

Differential Revision: https://phabricator.meteor.com/D827