Implement creating <font> tag

[goldenapples]

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 tag which is wrapped around the
  element's innerHTML.

[rauchg]

Needs tests. Looks good!

[thanj]

I can flesh this out with tests if you think it would be useful.

After trying to push forward with this approach, though, I've realized what an awful idea it really was. This hacks through some minor annoyances in Outlook, only to create a lot more problems in every other browser. (One example: you lose the ability to define font size accurately - <font> tags are limited to size=1 through size=6, so you lose any fine-grained control over type size and leading...)

As @jng5 reported on the issue #65 (comment), using ms conditionals to define web-safe fallback fonts for Outlook is a much more sane approach.

I'm in favor of closing this.

[rauchg]

Oh, that'd be really great to have, but potentially as another module? ie: juice-outlook.