Run juice even without a stylesheet so <style> tag CSS can be inlined.

I have the same request as https://github.com/niftylettuce/node-email-templates/issues/35.

What I've done here allows me to do something like this:

    <style>
      <% include ../shared-styles.css %>
    </style>

so that multiple email templates share CSS that gets inlined by juice. I implemented this in a weird, lazy way, so I understand you probably won't want to merge it without some cleaning up.

Are you willing to support something like this or come up with a better way to share CSS?