Add themes to style html emails

A new select input is added in the admin form for HTML theme to use; the list is taken from the deployed themes as per the ``qa_admin_theme_options()`` function and the default is the current site theme, as per ``qa_opt(site_theme)``.

The user may also use custom .css files, which would be added in the plugin's ``custom-styles/`` folder and selectable from the same input, with names ``Custom - <filename.css>``.

A ``<style>`` node is added to the footer of the email's html content, containing the contents of the entire ``qa-styles.css`` file of the configured theme.  The actual email's body content is wrapped around a ``<div class="publish2email-body">`` node (in order to override unwanted styling of ``<body>`` nodes from the CSS), which is given a default margin of 10px and left text-align.

The email body itself also now has all the minimum components of an HTML5 document, namely:

``` html
<DOCTYPE! html>
<html>
  <header></header>
  <body>
    <!-- email content wrapped with <div class="publish2email-body"> -->
  </body>
  <!-- optionally added if a style is selected/available -->
  <footer>
    <style>...</style>
  </footer>
</html>
```