EAD handler should handle mixed content in <p>

[bencomp]

Many CHIs use either <p>content</p> or <p><emph>content</emph></p> in elements like <bioghist> and <scopecontent>. Some, however, use mixed content, such as

<p>Content <emph>more content</emph> and then some more.</p>

If we mark both /p/ and /p/emph/ as 'include', we end up with a list with the content of <emph> first. If we only mark /p/ as 'include', the content of <emph> is lost.

The parser should support this mixed content and transform elements to what we want when it encounters them. (The ugly solution is to convert mixed content to Markdown equivalents before parsing, which is what I'll do for now.)

[bencomp]

The same applies to <extref> inside <accessrestrict>.