Support supplying only an htmlEntryPoint

[jakemac53]

Assuming there is only one dart script tag, and it lives in the same library as the html file, we can just use that dart file as the entry point and not require it to be specified:

transformers:
- initialize:
    entryPoint: web/index.dart
    htmlEntryPoint: web/index.html

becomes just:

transformers:
- initialize:
    htmlEntryPoint: web/index.html

[jakemac53]

we could also possibly just use entryPoint but allow you to pass an html file instead of a dart file, and then do the right thing based on the file extension

[sigmundch]

How about we do this by hiding this transformer inside another transformer in web_components?

If you write:

transformers:
- web_components/initialize:
    entry_point: web/index.html

[sigmundch]

ah- never mind, I thought initially this was for specifying the entry point for html-initialization, not just plain initialization. Now that I see #10, I see that this was just to update the bootstrap.html

[jakemac53]

fixed in 8ff3cf3