Add configs for TopoJSON files

[alrra]

Specification: https://github.com/topojson/topojson-specification

File extension: .topojson
Media type: Couldn't find any information about the recommended media type. Maybe simply go with application/json (or with the same media type as GeoJSON, namely, application/vnd.geo+json)?

Cc: @calvinmetcalf, @mbostock

Things that need to be done:

• set media type
• set expires headers
• ensure that TopoJSON files:
  • are served with the charset parameter that specifies that the character encoding of the documents is utf-8
  • are compressed (it's JSON, so it will compress quite nicely)
  • aren't served with all the HTTP headers intended only for HTML documents
• other (which?).

[calvinmetcalf]

since it isn't spec compliment geojson don't use that one, application/json is likely a safe bet for the moment, but I wouldn't be surprised if at some point application/vnd.topo+json or similar came around

[alrra]

Thanks @calvinmetcalf!