Command line (CLI) support for custom metadata

[dennisvang]

related to #99

Although it may be cumbersome and error prone, it would be useful to allow custom metadata to be specified via the command line.

Some options:

• allow only a pre-defined list of 'changes', using some custom format, e.g. separated by e.g. ;
• allow full-fledged JSON

This may be complicated by the way different shells handle quotes and escaping...