Async method alternatives are missing

[pallibj]

In using HtmlTextWriter to write directly to the response stream in asp.net core, those writes need to be async.

HtmlTextWriter does support some async methods to do writes, but it lacks async alternatives for methods like WriteFullBeginTag, WriteEndTag, WriteBeginTag and similar methods.

[mfeingol]

In general, my goal with this was to emulate the existing HtmlTextWriter and go no further.

However, if you'd like to submit a PR that adds async methods, I'll take it.

[pallibj]

Ok, thanks, no problem. I might do that later. For now I'll test adding it locally by inheriting your class. It it works well, I might submit a PR.