Customizer context

[bcampeau]

We should consider adding a Customizer context. It would support all Fieldmanager fields in the left column and would also support a template to load in the right column for preview purposes.

There is heavy development on the Customizer for 4.3, so this might be worth waiting to fully consider until after that release.

[mboynes]

This would be awesome. I think it's much trickier than it sounds, especially because the customizer has its own fields API. @dlh01 has done some one-off work on using FM with Customizer, he might be able to offer an informed take on it.

[danielbachhuber]

There is heavy development on the Customizer for 4.3, so this might be worth waiting to fully consider until after that release.

If you want to package as a standalone plugin for the time being, we would be happy beta testers :)

[dlh01]

All I did in that one-off work was borrow the data from some Fieldmanager objects to help construct Customizer objects. The data could have just as easily come from an array or something.

I'm starting a project in a couple weeks where I'd like to be able to try out an actual Customizer context. I'm interested in seeing what's possible.

[dlh01]

The project I mentioned above did end up creating something of a Customizer context. Once #380 goes in, I plan to start working on a proper context here.