Include name, public id and system id in Doctype event

[drott]

In some clients, such as web browsers, it is desirable to receive segmented information on name, public id and system id url with the Doctype.

Filing this issue to request an API change or API transition to have a Doctype event which includes these part as event properties.

[drott]

Thanks for review and merging. @kornelski, could you make a release for this and #56 ?

[drott]

Thanks for releasing 1.2.0.