Allow changing the vector tile style

[asitemade4u]

Currently Headway uses bright style but it would be nice to allow the style to be changed from the docker-compose.yml file, as an environment entry.

[ellenhp]

First I'd like to get one working map style (see: #47), but this is a good idea. :) We're actually using a fork of OSM Liberty though, it's just named like OSM Bright because I forgot to rename it when I changed the style.

[ellenhp]

This will be trickier than I thought so I'm adding a help wanted tag. If someone wants to tackle please PR it back :)

[lindleyw]

If this were possible, I would like to help create for example a config that would show railroads in heavy dark lines, trams and metros a contrasting color, and all roads in light grey.

[ellenhp]

In that vein, it would be very cool to allow headway to switch styles at runtime too.

The way I see it this will require a change to the tileserver init code to pull in style templates from a pre-generated artifact tarball, then run envsubst on each one, and save them to the correct location. After that, it will generate a tileserver configuration json file that will point tileserver-gl-light at each style. Once that's done, something will have to be done to point the frontend at the correct style. I don't really have ideas for that at the moment.