Implement creating <font> tag

As I brought up in #65:

Adds an additional option to the `options` object, called
`createFontTags`. If the option is set to "true", then while looping
through the document elements to create style props, changes the
behavior of the `setStyleAttrs` method:

- if the style attribute contains any of `font-family`, `font-size`, or
  `color`, those elements are not added to the style attribute, but
  instead are added to a <font> tag which is wrapped around the
  element's innerHTML.