add possibility of global _properties object in MML

[nebulon42]

References layers and their properties
fixes #450

I'm not yet sure which properties definition should be more important. Currently if both _properties and properties for a layer contain the same property properties wins. This cannot happen with TM2, but it is necessary to specify for carto.

@pnorman please try it out if it works for you.
cc @gravitystorm

[coveralls]

Coverage increased (+0.04%) to 89.135% when pulling 81404bb on nebulon42:global-properties into e7c4d2b on mapbox:master.

[pnorman]

@pnorman please try it out if it works for you.

How would I test this in Kosmtik?

[nebulon42]

You could checkout this PR somewhere and link the git repository into node_modules/carto in Kosmtik. Or maybe even easier specify "carto": "nebulon42/carto#81404bb4c86b888c2f40892c5f9d9f0ae0da40a8" in Kosmtik's package.json (see https://docs.npmjs.com/files/package.json#github-urls for details).

[pnorman]

Confirming that this works.