Add transformer option to rewrite original entry point script tag

[jakemac53]

Should be called something like htmlEntryPoint and take a file path to an html file. It will then search for script tags linking to entryPoint and change them to newEntryPoint.

[sigmundch]

Another option here, we could give the developer the option to replace the original entrypoint. If they don't have a Dart import to index.dart, we could move the original index.dart to original_index,dart and make the new index.dart the boostrap file with the new initializers.

[sigmundch]

Looking at #11, I don't think that the alternative option should be the only option, I think you are right that having the htmlEntryPoint is a good thing to have. What are your thoughts on also adding the other option though?

[jakemac53]

I don't think it makes a big difference which one we do, and I would rather only support one golden path just to reduce confusion. I like the idea of the bootstrap file a bit more just because it leaves all the original dart files completely in tact, so when debugging everything is a bit more intuitive.