Per-field provenance (source) information

[jpmckinney]

May require similar solution to #47

@rusosnith @davewhiteland @sweemeng

[sweemeng]

Currently I am hacking the link field with an optional source in popit_ng. I am thinking a separate field is not a bad idea.

Here is a few decision I have made when designing the system, even though it ends up being adding 1 field

• Multiple source is fine. It make a field value stronger
• it should have enough detail on the source, which is why we reuse links
• field must exist, this is integrated inside app not on data.