Allow the date/time formats to be set.

Thes can be read from the configuration file, and this updates
Document date/time setting.

This allows format-changes to be applied to generated blogs
as per #28.
Updated the way `getBlog` is called.

This is required such that we can allow the date/time format
strings to be read from the configuration file in the
`chronicle::getBlog()` method.  Unfortunate to change the API
like this but low-risk.