[gulp] start script improvements

[pkra]

Some stray thoughts on potential improvements.

• When using npm start and then something like http://localhost:4001/?url=path/to/file.xml, the assets for file.xml are not loaded because the URLs are not made relative to the file's location. It would be nice to fix that.
• Add CLI option
  • It would be convenient to have something like npm start path/to/file.xml work.