Proposal - Allow mixed case attribute names

[jmooring]

Currently, this does not do anything:

```text {LineNos=true LineNoStart=42}
foo
```

You must do this instead:

```text {linenos=true linenostart=42}
foo
```

Users are accustomed to case-insensitive HTML attribute names. I understand this is markdown, not HTML, but there is a similar expectation from non-developer users.

Reference:

• Proposal - Allow mixed case attribute names goldmark-highlighting#30

[yuin]

FYI, pandoc does NOT allow this kind of 'relaxed' attribute names.

~~~~ {#mycode .haskell .numberLines startFrom="100"}
qsort []     = []
qsort (x:xs) = qsort (filter (< x) xs) ++ [x] ++
               qsort (filter (>= x) xs)
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

works,

~~~~ {#mycode .haskell .numberLines startfrom="100"}
qsort []     = []
qsort (x:xs) = qsort (filter (< x) xs) ++ [x] ++
               qsort (filter (>= x) xs)
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

not works

[jmooring]

Sure, but that doesn't mean it's the right thing to do.