GitHub renders ~~foo~~bar as foobar and foo~~bar~~ as foobar.

pulldown-cmark however doesn't recognize both strikethroughs:

$ echo '~~foo~~bar' | cargo run  -- -S
<p>~~foo~~bar</p>
$ echo 'foo~~bar~~' | cargo run  -- -S
<p>foo~~bar~~</p>

Unfortunately the GFM spec doesn't say much about strikethroughs (see github/cmark-gfm#74), but we could of course still be consistent with the established GitHub rendering.