Unwrap dynamically imported modules

[lmartins]

@estrattonbailey Hi Eric, sorry it took me a while to get to this, but played with a small change that I think is working pretty great to allow Webpack's dynamic imports.

This commit checks if the loaded instance is a promise, unwraps it if it is, and then instantiate the object as picoapp normally would.

https://github.com/estrattonbailey/picoapp/issues/7

Thank you!