[TIMOB-20546] Adding opacity property for fillColor in ti.map.polygon

Steps to Reproduce

var MapModule = require('ti.map');
var polygon = MapModule.createPolygon({
   fillColor: '#123456',
   strokeColor: '#123456',
});

Actual Result

Actually there is no property to which would allow to change the opacity of fillColor of polygon. The only way to achieve that is a workaround with alpha channel for fillColor property, which is not described anywhere:

   strokeColor: '#88123456',

It is not possible with name values of colors like :'red', 'blue'

Expected Result

Polygon disctionary should allow setting opacity for fillColor.

var MapModule = require('ti.map');
var polygon = MapModule.createPolygon({
   fillColor: '#123456',
   strokeColor: '#123456',
   fillColorOpacity: 0.5
});