What problem does this feature solve?

Since version 2.1.0 an error is thrown when you try to run without the --bundle option. In some projects, choice has been made to not use webpack cause of incompatibility with specific module (you can avoid to use webpack by exporting templates in js file instead of the usual vue files).
It would be great to have a way to discard this requirement.

What does the proposed API look like?

Sorry, I don't have precise idea how to implement this feature.