allow configuration of more metadata fields

[mattkime]

in my current internationalization project i'm focusing on clear communication between translators and devs. it would be helpful if we could define our own metadata fields that would be treated like the description field - copied along with id and defaultMessage into the extracted json files but otherwise unused during rendering. We wish to have a field that specifically references our ticketing system.

The ideal setup would be to make babel-plugin-react-intl configurable so we could define which fields we want treated with metadata, with the default being [ 'description' ].

Having taken a look at the code this appears to be a simple change affecting ~10 lines of code or so. I wanted to start the conversation to see if this would be a welcome change before submitting a pr.

Thoughts?

[ericf]

Yeah this seems to make sense to make the metadata concept more generic and treat it similar to the description field which has no runtime semantic or impact. Feel free to propose a PR with this idea more fleshed out.

Did you think about parsing your descriptions after they've been extracted and then expanding the object you send to the translators with this additional metadata?

[mattkime]

Yes, we could parse the description field and may do that in the short term but intent is more clear if we have separate fields.

I'll follow up with a PR.

[ericf]

Merged #87