Add "Credit Line" (IPTC core 1.1)

[hyanwong]

The IPTC site says

Note 2: This property was named "Credit" by the IIM metadata, then it was renamed to "Provider" in IPTC Core 1.0. In IPTC Core 1.1. it has been renamed to "Credit Line" as the field is used for this purpose by many users.

"credit line" is not present in https://github.com/jamesacampbell/iptcinfo3/blob/9542c100e8b0fde171abc6ef64616fb9097d3b61/iptcinfo3.py#L446. How does iptcinfo3 cope with IPTC Core 1.1?

[james-see]

Not supporting this. You can add a custom tag as you wish, which makes it flexible. Until this is sponsored, I just have no time.

[james-see]

Adding help wanted tag and keeping open, thanks for the request on this. I want to support updating to comply with IPTC Core 1.5x and newer eventually.