demo mode

[burritojustice]

per the old branch, a very simple way to toggle between a subset of tags and properties, minimal UI.

Initially thought we could define the demo as a blob of json — demo=chicago.json uncertain where that would live though.

[burritojustice]

see also #22

[bcamper]

Can probably build off of and/or reuse the mobile view styles, but in a non-mobile context.

[burritojustice]

changes are looking good! we should also add the other properties we're analyzing other than color -- point size width, labels (and other future property-driven features)

Analyzing points by X
Scaling points by Y
Labelling points by Z

[burritojustice]

we should add these to the pop-up menu(s) as well