add blockquote markup

I added a simple blockquote handler for using the "underscored" quote style:

    [quote, Some Person, Some Source]
    _____________________________
    blah, blah, blah
    
    other paragraph
    _____________________________

which is rendered as:

    <para>
      <blockquote>
        <para>blah, blah, blah</para>
        <para>other paragrah</para>
      <attribution>
        <para>Some Person</para>
        <emphasis>Some Source</emphasis></attribution>
      </blockquote>
    </para>

This validates against the Manning schema as checked by `xmllint`.