Added option to set the publish date of a post

Previously, Static used an item's created date as its publish date. This
led to inaccurate publish dates when a post was created and not
immediately published. As a workaround, users could edit the plist
associated with the post's file to change the created date, but this was
cumbersome.

This commit allows users to specify the publish date by adding a date
string meta value (in `%Y-%m-%d %H:%M:%S` format) under a page's
`pubdate` meta key. This is treated as an override; if the meta value is
not found or cannot be parsed, the page generation code reverts to the
`createdDate`.