Strikethrough fails GFM spec 493

[chrisjsewell]

As per https://github.github.com/gfm/#example-493, and demonstrated below:

this is ~~indeed~~ a strikethrough
this is ~~~not~~~ a strikethrough

this is indeed a strikethrough
this is ~~~not~~~ a strikethrough

markdown-it creates a strikethrough for the three tildes