Implement "has" / "!has" filter operators

[jfirebaugh]

ref mapbox/feature-filter#15

cc @1ec5 @jfirebaugh @mourner

[mb12]

@lucaswoj What is the difference between "has" and "in" operators?

[lucaswoj]

@mb12 "has" returns true if a feature provides a value for key. in returns true of the value for key is in a provided set of values.

['has', 'mapbox'] // returns true if the feature has a value for "mapbox"

['in', 'mapbox', 'neat', 'swell', 'great'] // returns true if the value for "mapbox" is 'neat', 'swell', or 'great'

[lucaswoj]

It is similar to but not quite the same as ['==', 'mapbox', undefined]