function parseAuthOptions (opts) {
  var matches
  if (opts.auth) {
    matches = opts.auth.match(/^(.+):(.+)$/)
    if (matches) {
      opts.username = matches[1]
      opts.password = matches[2]
    } else {
      opts.username = opts.auth
    }
  }
}
Parse the auth attribute and merge username and password in the options object.

@param {Object} [opts] option object