handle <subtitle> inside <cite> in FictionBook documents
According to FictionBook XSD, it is valid:
<xs:complexType name="citeType">
<xs:annotation>
<xs:documentation>A citation with an optional citation author at the end</xs:documentation>
</xs:annotation>
<xs:sequence>
<xs:choice minOccurs="0" maxOccurs="unbounded">
<xs:element name="p" type="pType"/>
<xs:element name="poem" type="poemType"/>
<xs:element name="empty-line"/>
<xs:element name="subtitle" type="pType"/>
<xs:element name="table" type="tableType"/>
</xs:choice>
<xs:element name="text-author" type="pType" minOccurs="0" maxOccurs="unbounded"/>
</xs:sequence>
<xs:attribute name="id" type="xs:ID" use="optional"/>
<xs:attribute ref="xml:lang"/>
</xs:complexType>