Create a customStyle api for document custom properties

[sorvell]

An element can set a custom style property via this.customStyle['--foo'] = 'red'; but there is currently no way to do this in the document scope. Currently, a <style is="custom-style"> element must be created and added to the document. It would more convenient for example, to do this: Polymer.StyleDefaults.customStyle['--foo'] = 'red';

[sorvell]

Fixed via #1916