Poll react-native community regarding Metro plugins

[afoxman]

While building the Metro plugin proposal #1290, solicit react-native community feedback.

Do engineers want this?
What would they do with it?
Should that functionality be added to Metro instead of exposed as a plugin?
Would devs be willing to write a plugin, or adapt one from other bundlers like webpack?

Use this data to refine the proposal and create a detailed engineering plan.