Implement `{: data-line="1"}` for setting block-level attributes

[kjk]

## foo
{: data-line="1"}

should generate:

<h2 data-line="1" id="foo">
  foo
</h2>

babelmark results

This is a kramdown extension. Probably should be an extension.

[miekg]

[ Quoting @.***> in "[gomarkdown/markdown] Implement `{:..." ]

foo

{: data-line="1"}

babelmark results

This is a kramdown extension. Probably should be an extension.

likely. Note that for mmark we already support such things (diff. syntax though, yeah! markdown)

[miekg]

This would be hacking on the Attributes I've added a while back, but now they are detected by { data-line="1"}

[miekg]

also kramdown allows these after block level elements, haven't been able to do that here...