Add -recordkey flag to marctojson.

The record key can be specified by the user.

The default is now `record`; before, it was hardcoded `content`.

To emulate old behaviour:

    $ marctojson -recordkey content file.mrc
Merge branch 'miku-content-option'

* miku-content-option:
  Add -recordkey flag to marctojson.
marctools 1.6.0

* adjust namespace from miku to ubleipzig
Adjust Makefile repository urls.