Add color to built-in entity extraction

[ericzon]

Is your feature request related to a problem? Please describe.
It's a typical feature that would be very useful to have built-in inside this great lib. As @sys.color in DialogFlow https://dialogflow.com/docs/reference/system-entities

Describe the solution you'd like
Extract colors of an input.

i.e:

"I have a red car"

Output:

{
	"locale": "en",
	"localeIso2": "en",
	"language": "English",
	"utterance": "I have a red car",
	"classification": [{
		"label": "color",
		"value": 0.8567240019144264
	}],
	"intent": "car.getcolor",
	"domain": "default",
	"score": 0.8567240019144264,
	"entities": [{
		"start": 9,
		"end": 11,
		"len": 3,
		"levenshtein": 0,
		"accuracy": 1,
		"option": "color",
		"sourceText": "red",
		"entity": "general",
		"utteranceText": "red"
	}],
	...,
	"srcAnswer": "ok, we took note of your color",
	"answer": "ok, we took note of your color"
}