px:set-doctype - sets a custom doctype

Allows you to set the doctype to '<!DOCTYPE html>', or whatever else you want.

Also added a "doctype" attribute to the `d:file` elements that are parsed
by `px:fileset-store`. If `p:store` is used to store XML, and "doctype" is set,
then `px:set-doctype` will be invoked on the stored file.

So, to store (X)HTML5-documents with `<!DOCTYPE html>` now...:
`<d:file href="content.xhtml" doctype="&lt;!DOCTYPE html&gt;" ...>`

fixes daisy/pipeline-issues#386