Please add strikethrough and underline support in Markdown

Standard HTML code strike-though is not yet available! now available!

According to the HTML5 spec, <s> or <strike> serve a different purpose to <del> semantically.

http://www.w3.org/TR/html5/text-level-semantics.html#the-s-element

The <s> element represents contents that are no longer accurate or no longer relevant.

NOTE: The s element is not appropriate when indicating document edits; to mark a span of text as having been removed from a document, use the del element.

http://www.w3.org/TR/html5/edits.html#the-del-element

The del element represents a removal from the document.

Using <del> also causes rise for people to use <ins> to insert text, the underline text on the first line of this post is inserted like this:
<ins> now available!</ins>