support delimiters with length above 1

[arve0]

Setup:

md.use(attrs, {
  leftDelimiter: '{{{',
  rightDelimiter: '}}}'
})

Markdown:

this is the markdown I'm trying to parse {{{.replace-me}}}

Current output:

<p>this is the markdown I'm trying to parse {{{.replace-me}}}</p>

Expected output:

<p class="replace-me">this is the markdown I'm trying to parse</p>