Support multiple input files

[GretaCB]

Per https://github.com/mapbox/tilelive-omnivore/issues/26

cc @BergWerkGIS @rclark

[GretaCB]

Tested locally and XML results look :+1: ! Also did a quick test through tilelive-bridge

Thinking updating the Readme is a great next step (for example: bin/mapnik-omnivore usage log and usage example for new Omnivore(uri...) )

---

Just noting for myself for later when I forget :smile: :

• uri for multi-file will be a comma separated string

var uri = 'omnivore:///Users/data/geojson/DC_polygon.geo.json,/Users/data/geojson/places.geo.json'

new Omnivore(uri, function(err....)};

• mapnik-omnivore stays the same, and will return a separate metadata object for each file. tilelive-omnivore will then handle these within an array:

[ { filename: 'DC_polygon.geo',
   ...
    json: { vector_layers: [Object] },
    layers: [ 'DC_polygon.geo' ],
    filepath: '/Users/data/geojson/DC_polygon.geo.json' },
  { filename: 'places.geo',
    ...
    json: { vector_layers: [Object] },
    layers: [ 'places.geo' ],
    filepath: '/Users/data/geojson/places.geo.json' } ]

[coveralls]

Coverage increased (+4.6%) to 97.101% when pulling abc58f550cb6fb15cecdc42b18239c66b3f25c1e on multiple-input-files into ba1d60fdaf01713888877c44b9a1ec2911ec49e1 on master.