Add linestring viz

[ryanbaumann]

Create a new Viz class tyle, LinestringViz, that allows a user to create a line map from a geojson source or a vector tile source. Follow the same techniques used to make the Choropleth Viz. #40

Example:

[akacarlyann]

@ryanbaumann in terms of styled elements of a LineStringViz, I'm thinking color and width could both be data-driven. This is slightly different from the Choropleth approach, which defines a single line color/stroke/width for the entire map. Anything else you want to add? I can open a PR this afternoon for this.

[ryanbaumann]

Agree with color and width being the two major features to control, @akacarlyann. It's similar to the Choropleth, just Choropleth uses color and height, while Linestring uses color and width. Obviously excited to review a PR, if you have one!

[akacarlyann]

Rough start at #92. Found I might want to generalize mapboxgl.utils.height_map from use in ChoroplethViz first too.

[ryanbaumann]

Great @akacarlyann. We can open up a separate PR to address the utils function sprawl before the next release. Agree we need to keep the utils functions as generic and re-usable across visualizations as possible.

[ryanbaumann]

Closed in #92