Accept function in legacy _template field for template parsing

[kevinpschaaf]

Polymer's _template field will now accept a template-returning function in addition to a concrete HTMLTemplateElement, to allow for lazy template parsing.

Example:

Polymer({
  is: 'lazy-template',
  _template: () => html`<div>lazy template</div>`;
});

In addition to the previously supported way:

Polymer({
  is: 'eager-template',
  _template: html`<div>eager template</div>`;
});

Reference Issue

Fixes #5660