Add support for properties with type "exists"

[diegoquinteiro]

Support properties with type exists like related.sponsored below:

{
	"class": "RelatedItemRule",
	"selector": "li",
	"properties": {
		"related.sponsored": {
			"type": "exists",
			"selector": "li",
			"attribute": "data-sponsored"
		},
		"related.url": {
			"type": "string",
			"selector": "a",
			"attribute": "href"
		}
	}
}