Add tag and contentTag methods to HtmlBuilder

This pull request implements the feature mentioned in Proposal #5087. See the proposal for more detailed information.

It adds a tag and contentTag method to the HtmlBuilder class, and also adds a few helper methods to make them easier to use in views. Although I didn't do it in this pull request, HtmlBuilder and FormBuilder could be rewritten to use these additions to simplify the existing methods like `link`, `image`, etc. I think this would be an improvement over the manual create of html tags in each method.