Add support for `AgeRating` column

[robrich82]

Can you please add support for AgeRating?

I share comics with my kids, currently I am using tags in Calibre but it would be nicer to directly support the AgeRating in ComicInfo.XML

<xs:simpleType name="AgeRating">
    <xs:restriction base="xs:string">
        <xs:enumeration value="Unknown"/>
        <xs:enumeration value="Adults Only 18+"/>
        <xs:enumeration value="Early Childhood"/>
        <xs:enumeration value="Everyone"/>
        <xs:enumeration value="Everyone 10+"/>
        <xs:enumeration value="G"/>
        <xs:enumeration value="Kids to Adults"/>
        <xs:enumeration value="M"/>
        <xs:enumeration value="MA15+"/>
        <xs:enumeration value="Mature 17+"/>
        <xs:enumeration value="PG"/>
        <xs:enumeration value="R18+"/>
        <xs:enumeration value="Rating Pending"/>
        <xs:enumeration value="Teen"/>
        <xs:enumeration value="X18+"/>
    </xs:restriction>
</xs:simpleType>