Proposal - Improve attribute parsing

[jmooring]

Users are accustomed to quoting HTML attributes, regardless of type, because many have no concept of type. And the browser doesn't care. I understand this is markdown, not HTML, but there is a similar expectation from non-developer users.

For example, when using goldmark-highlighting, quoting or not quoting an attribute makes a big difference.

Example	Notes
{linenos="false"}	Effectively setting it to true
{hl_lines="[2]"}	Ignored

Reference: yuin/goldmark-highlighting#28

[yuin]

IMHO, I do not stand for this. I have been had many troubles related this kind of 'relaxed' binding.