Custom url

[aparlato]

Closes https://github.com/stamen/chartographer/issues/33

Adds the ability to pull in a style from a URL. It also adds polling of that style if changes are made. @ebrelsford I know we discussed handling polling as a followup to this, but this reuses a lot of logic from Maperture and felt straight forward to leave in. If there's a lot of issues I didn't notice with polling, I'm open to removing and handling as followup instead.

Review/QA

• [ ] Existing drop in style functionality works as expected
• [ ] Add a style via URL (try both clicking the button and Enter key to submit)
• [ ] Confirm style appears and url remains in text box
• [ ] Drop in a different style while that one is up
• [ ] Expect new style to appear and text box to go blank
• [ ] Add a locally hosted style via URL
• [ ] Expect this to look like any other style
• [ ] Make a change to the live style
• [ ] Expect chartographer to reload the style with changes

[aparlato]

Sounds good! Just opened a followup here: https://github.com/stamen/chartographer/issues/48