Add support for importing srcbooks from the hub

[benjreinhart]

This PR:

1. Introduces a more robust CLI with support for importing srcbooks from https://hub.srcbook.com.
2. Removes the hardcoded localhost urls in the web package, which now means that we can configure port and leverage localhost aliases

CLI Examples:

npx srcbook start
npx srcbook start --port 5000   # or -p

npx srcbook import <hub-id>
npx srcbook import <hub-id> -p 5000  # or --port

Main CLI help menu

Usage: srcbook [options] [command]

Srcbook is a interactive programming environment for TypeScript

Options:
  -V, --version                 output the version number
  -h, --help                    display help for command

Commands:
  start [options]               Start the Srcbook server
  import [options] <specifier>  Import a Srcbook
  help [command]                display help for command

import command help menu

Usage: srcbook import [options] <specifier>

Import a Srcbook

Arguments:
  specifier          An identifier of a Srcbook on hub.srcbook.com

Options:
  -p, --port <port>  Port of the server (default: "2150")
  -h, --help         display help for command