Added --url option to iptest

Allows `iptest js` to be used against remote servers.

Example usage: `iptest js --url='http://localhost:8888'`

Pinging @rgbkrk who requested this.
Code style point, I think we should have parentheses here so it's obvious whether it's `a || (b+c)` or `(a||b) + c`, without having to think about JS operator precedence.