Allow bypassing babel

It is possible to write TS code such that Babel is not required to post-process TS compilation results. It would be a nice perf win for people in those situations if they could somehow prevent babel from being used.

For this feature, it doesn't make sense to inspect configuration options in tsconfig because they are not concrete enough to reliably express user intentions regarding babel usage.

I'm thinking that the best thing to do in the long run would be to expose an api to allow users to wrap and configure the transformer. Then they could point the bundler to their own transformer implementation.