Use CLOSED log drawer info if available to set the date in front-matter

[kaushalmodi]

For example, if a post has:

** DONE My post
   CLOSED: [2017-09-11 Mon 14:00]

the EXPORT_DATE property should not be needed at all as the date field of the front-matter will be derived from the CLOSED time-stamp.

[kaushalmodi]

@jave