Idea: CLI supporting raw filters json

[GretaCB]

Per @springmeyer 's thoughts:

The CLI currently expects a --style flag that reads a mbgl style that is then run through /lib/styleToFilters.js

I think it would be ideal to also:

• Have the CLI accept an option like --filters that would be the JSON schema which new shaver.Filter expects (in place of --style)
• Have this "Filters" JSON schema clearly documented in the readme so that it is obvious how to use shaver without needing to form up a full mbgl style.

This could be useful for testing and finding lurking parsing bugs.

refs

vtshaver/src/filters.cpp

Line 42 in da0b5ba

NAN_METHOD(Filters::New) {