message: Add intellisense for labels with one optional arg
  The command \label[<type>]{<label>} is defined by the cleveref package.
diff: @@ -168,11 +168,21 @@ export class Reference implements IProvider {
  if (latexParser.isLabelCommand(node) && node.name === 'label') {
  // \label{some-text}
  label = node.label
  - } else if (latexParser.isTextString(node) && node.content === 'label=' && useLabelKeyVal && nextNode !== undefined) {
  + } else if (latexParser.isCommand(node) && node.name === 'label'
  + && node.args.length === 2
  + && latexParser.isOptionalArg(node.args[0])
  + && latexParser.isGroup(node.args[1])) {
  + // \label[opt_arg]{actual_label}
  + label = latexParser.stringify(node.args[1]).slice(1, -1)
  + } else if (latexParser.isTextString(node) && node.content === 'label='
  + && useLabelKeyVal && nextNode !== undefined) {
  // label={some-text}
  label = latexParser.stringify(nextNode).slice(1, -1)
  }
  - if (label !== '' && (latexParser.isLabelCommand(node) || latexParser.isTextString(node))) {
  + if (label !== '' &&
  + (latexParser.isLabelCommand(node)
  + || latexParser.isCommand(node)
  + || latexParser.isTextString(node))) {
  refs.push({
  label,
  kind: vscode.CompletionItemKind.Reference,
  