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.
@crynobone Thanks for the heads up, I added another method parameter called escape that defaults to true and escapes content. If the user passes false to this they can make it so the content isn't escaped if they want to use this for nested content_tags or something like shown in the tests.
https://github.com/laravel/framework/pull/5089/files#diff-c4556b0f51113657877a73611cea9effR401