feat(ngModel): Treat the values of number and range inputs as numbers

Add a InputNumberLikeDirective that binds the model as a numeric
value, so that it can be bound to num types on scopes. Select the
directive for number and range input types.

Closes #527
refactor(expression-extractor): adapt for pub transformers

Slight refactoring of expression extractor to make it easier to drive from Dart transformers.

Primary change is exposing parseHtml on html_extractor to allow externally driving it rather than using a crawler.

Closes #537