Bug: MarkdownImport does not escape plus signs in the provided tag

[tehapo]

I was trying to use ++ as the tag for underline when converting to and from Markdown by adding a custom TextFormatTransformer. Unfortunately this fails with an invalid regular expression error.

Looking at the source code, it seems to me like this could be fixed by escaping the plus sign in MarkdownImport, but I don't know if there are any unwanted side-effects of doing that.

// This...
const tagRegExp = tag.replace(/(\*|\^)/g, '\\$1');

// ...should also include the plus sign?
const tagRegExp = tag.replace(/(\*|\^|\+)/g, '\\$1');

This is the TextFormatTransformer I was trying to use.

{
  format: ['underline'],
  tag: '++',
  type: 'text-format'
}

[fantactuka]

Sounds reasonable, feel free to create a PR! 👍