Support metadata streams (XMP)

[Heinenen]

A feature that is currently missing in lopdf is the ability to read/write metadata streams.
While some document metadata is already included in the "Document information dictionary", its use is discouraged and marked as deprecated in PDF2.0 (except for the entries CreationDate and ModDate).

The relevant part of the spec is "14.3 Metadata" and further ISO 16684-1 which describes the XMP format.