Support non-semantic tags for underline, strike-through and teletype text

[waldyrious]

Currently there are variants for two of the inline formatting markers (* and _ for <strong> and <em>, respectively), that allow styling text in the same manner but without the semantic meaning that the primary markers are meant to convey — i.e., as bold (<b>) and italic (<i>).

I believe it is reasonable to expect that the same behavior would also apply to the other formatting options that HTML supports in both semantic and stylistic variants, namely <ins>/<u>, <del>/<s>, and <code>/<tt>:

semantic (single markers)	stylistic (double markers)
*foobar* → <strong>foobar</strong> ✅	**foobar** → <b>foobar</b> ✅
_foobar_ → <em>foobar</em> ✅	__foobar__ → <i>foobar</i> ✅
+foobar+ → <ins>foobar</ins> ✅	++foobar++ → <u>foobar</u> ❌
-foobar- → <del>foobar</del> ✅	--foobar-- → <s>foobar</s> ❌
@foobar@ → <code>foobar</code> ✅	@@foobar@@ → <tt>foobar</tt> ❌

IMO Textile would be more consistent and predictable if all these combinations behaved the same way.

[waldyrious]

For reference, here's how some of the existing implementations currently render these:

Textile	textile-lang.com	textile.js	try.redcloth.org
--foobar--	&#8212;foobar&#8212; —foobar—		<del>-foobar-</del> -foobar-
++foobar++	++foobar++ ++foobar++
@@foobar@@	<code></code>foobar@@ foobar@@	<code>@foobar@</code> @foobar@	@@foobar@@ @@foobar@@

Even if the extension proposed above isn't adopted, at least the double-marker behavior could be explicitly specified so it is consistent. For example, either of the two alternatives below would improve the consistency of all the implementations above:

1. Ignoring the double markers as Textile syntax;
   (as currently done by try.redcloth.org for --)
2. Recognizing the outer markers as Textile syntax, and leaving the inner ones untouched.
   (as currently done by textile.js for @@, and all three for ++)

Textile	HTML (option 1)	HTML (option 2)
--foobar--	--foobar--[1]	<del>-foobar-</del>
++foobar++	++foobar++	<ins>+foobar+</ins>
@@foobar@@	@@foobar@@	<code>@foobar@</code>

^{1. optionally with typographical enhancement, i.e. "—foobar—"}

[waldyrious]

Since this repo hasn't seen much activity lately, I'm taking the liberty to ping @philwareham, who's been actively maintaining the textile/textile-lang-com-website repo.