Add otpauth to allowed schemes for anchor elements

[dberry-trivir]

Fixes issue #25

[andrewhertel]

As this is purely a TextOutputCallback use case, we might want to scope it, e.g., in sanitizerConfig.js, we can add

export const textOutputCallbackSanitizerConfig = {
  ...baseSanitizerConfig,
  allowedSchemesByTag: {
    ...baseSanitizerConfig.allowedSchemesByTag,
    a: [...sanitizeHtml.defaults.allowedSchemes, 'otpauth'],
  },
};

And then we can import textOutputCallbackSanitizerConfig and use it directly in the TextOutputCallback.