Provide a convenience method for setting `customStyle` and calling `updateStyles`

[sorvell]

Polymer provides a customStyle object which can be used to set css custom properties. To apply these values, updateStyles must be called. Since this is typically done in lockstep, there should be a 1 step option, e.g.:

this.updateStyles({'-foo': 'red'});
// short for
this.customStyles['--foo'] = 'red';
this.updateStyles();