Add support for actually utilising custom Feed Me Data Types.

[timeverts]

Josh,

While doing some experimenting with Feed Me, I needed to create a custom data type but then could not work out how I could actually create a feed with that custom data type. There was no built in way to select my custom data type for a new feed even though it was registered using the registerFeedMeDataTypes plugin hook. It would also not be possible to save a new feed with a custom data type, since the feeds record is restricted to your built in types.

So I went about making some changes to properly support custom Feed Me Data Types. Take a look and see what you think.

Note that I added the functionality to allow specifying a different display name for a custom data type so that the default displayed name of MyPlugin_MyCustomFeedMeDataType can be customised.

Let me know if you have any questions or recommendations.

[engram-design]

Thanks for setting this up. There are a few tweaks I might do to this, but for the most part, on the right track. Third-party data types were a bit rushed in the end, and was going to address it properly when someone had the need for it.

I'll hold off on adding this for the moment, to bundle with the next release as it'll change the schema. Thanks!

[engram-design]

@timeverts Thanks again so much for the PR (not to mention the migration), its now been rolled into 2.0.2 (didn't have to change a thing).