Figure out how to package reusable assets

[thbar]

Types of assets:

• commonly used sources, transforms and destinations (e.g.: CSVSource)
• syntactic sugar like show_me!, limit(10) or field_transform(xxx)

These could be either part of kiba itself, or a kiba-common gem, or maybe separate repositories, sometimes provided by 3rd parties.

[thbar]

I now know very clearly how to do that (based on real-life experience). Will document this in #29. Closing.