Strikethrough extension is underspecified

[srawlins]

As far as Markdown-enabled textboxes on GitHub are concerned, the Strikethrough extension is underspecified. In general, it seems to have some of the same left-flanking/right-flanking rules. I haven't looked at the code yet, but here are some examples. According to the spec:

Strikethrough text is any text wrapped in tildes (~).

However, we can find a number of counter examples:

A "right-flanking" ~ cannot open text, nor can a "left-flanking" ~ close strikethrough

Markdown input	as rendered by GitHub
~ text~	~ text~
~text ~	~text ~

The "multiple of 3" combined delimiter rule seems to apply

From the last sentence of rule #9.

Markdown input	as rendered by GitHub
~foo~~bar	~foo~~bar
~~foo~bar	~~foo~bar

The shorter span rule seems to apply

From rule #16

Markdown input	as rendered by GitHub
~~foo ~~bar~~	~~foo bar

The "links group more tightly" rule seems to apply

From rule #17

Markdown input	as rendered by GitHub
~[foo~](bar)	~foo~

I'd be happy to send a PR.

[puzrin]

Probably, partially solved by #99