Allow for specifying the port

[pillowfication]

Currently dev-harness only runs on port 8080 and can't be changed. This is annoying when trying to develop two plugins side-by-side, or if port 8080 is already used by something else.

Webpack already allows specifying the port (e.g. npm start -- --port 1234), but 8080 is hardcoded into the Adapter.

<iframe
  ref={this.handleRef}
  src="http://localhost:8080/childApp.html"
  style={{ border: 0, height: '100%', width: '100%'}}
  title="child-app"
/>