Add PDF metadata parsed from HTML. Fix #77.

`<title>` → /Title
`<meta name=author>` → /Author
`<meta name=description>` → /Subject
`<meta name=keywords>` → /Keywords
`<meta name=generator>` → /Creator
`<meta name=dcterms.created>` → /CreationDate
`<meta name=dcterms.modified>` → /ModDate
"WeasyPrint vX.Y" → /Producer