Set Create Date metadata

[raffaem]

Following #297 , can we have an option to set the "Create Date" metadata?

[u-fischer]

there are the (undocumented) keys pdfcreationdate and pdfmoddate. I have extended the documentation:

The dates |CreationDate| and |ModDate| 
 are normally set automatically to the current date/time when the compilation
 started. If they should be changed
 (e.g. for regression tests to produce reproducible documents) they can
 be set with |\hypersetup| with the keys
 |pdfcreationdate|, |pdfmoddate| or externally by setting the |SOURCE_DATE_EPOCH| environment variable. 

 \begin{verbatim}
 \hypersetup{pdfcreationdate=D:20010101205959-00'00'}
 \end{verbatim}

 The format should be a full date/time in PDF format, so one of these (naturally
 the numbers can change):
 \begin{verbatim}
  D:20010101205959-00'00'
  D:20010101205959+00'00'
  D:20010101205959Z
 \end{verbatim}