mkpost.go

Unlike some of the other functionality provided by Logkit,
the logging functionality does not wrap an underlying log provider.

Why not? Because, in a past life, I worked as a Java developer and
therefore understand that log wrappers will bring you nothing but pain
and unhappiness.

Instead of being a log wrapper, this functionality encourages
applications to use logrus. Logrus is already widely used in GitLab.

What benefit does this functionality add:
* Consistent configuration of logrus
* Importantly, this functionality allows labkit itself to write logs.
  * This will allow us to add better logging, as well as standardised
    access loggers, exception loggers etc

Co-authored-by: Zeger-Jan van de Weg <zegerjan@gitlab.com>