Support multibyte character in heading id

[kenchan0130]

Marked version:
0.3.18

Markdown flavor:
n/a

Markdown

# これは markdown です

Expectation

<h1 id="これは-markdown-です">これは markdown です</h1>

or

<h1 id="これはmarkdownです">これは markdown です</h1>

Result

<h1 id="-markdown-">これは markdown です</h1>

Cause

It seems that multibyte characters have been converted to hyphens in the processing of this part.

[UziTech]

A PR would be welcome

[UziTech]

according to W3C id spec:

When specified on HTML elements, the id attribute value must be unique amongst all the IDs in the element’s tree and must contain at least one character. The value must not contain any space characters.

but this will be a breaking change.

/cc @Feder1co5oave

[UziTech]

#981

I'm going to close this as duplicate. There are many different takes on generating header ids. Some people want hyphens combined when there are multiple in a row, some people don't. Some people want hyphens trimmed when they are at the beginning or end, some people don't.

Header ids are something we are looking at deprecating in the future so people can use their own algorithm to generate them.