Add image export function to viz classes

[ryanbaumann]

A common data science workflow is to include a static image version of a visual for sharing or screen shot creating.

This could be added as a saveImage() HTML button on a viz class object that downloads the current map viewport as a PNG, or it could be a viz.toImage() python function that outputs a PNG of the viz at the stated zoom, center, pitch, and bearing.

[ryanbaumann]

Related to #28

[ryanbaumann]

Adding a help-wanted label here - I'd love to get this feature into a 1.0 release (Targeting end of February 2018) to allow distribution of Notebooks with lightweight static visualizations

[akacarlyann]

Is there a way to leverage https://www.mapbox.com/help/static-api-playground/ without going through mapbox studio?

[ryanbaumann]

@akacarlyann the Mapbox Static API would work for small data use cases, but not large ones. The good news is that rendering an image from a gl context (which is the rendered Mapboxgl map) is pretty straightforward - for this use case, if we're running in Jupyter I'm envisioning just a button you can push to save the rendered map to a JPEG or PNG file. Supporting a headless python function to render the map would be the next step, which is a fair bit more work.