get subdomains () {
    const offset = this.app.subdomainOffset
    const hostname = this.hostname
    if (net.isIP(hostname)) return []
    return hostname
      .split('.')
      .reverse()
      .slice(offset)
  }
Return subdomains as an array.

Subdomains are the dot-separated parts of the host before the main domain
of the app. By default, the domain of the app is assumed to be the last two
parts of the host. This can be changed by setting `app.subdomainOffset`.

For example, if the domain is "tobi.ferrets.example.com":
If `app.subdomainOffset` is not set, this.subdomains is
`["ferrets", "tobi"]`.
If `app.subdomainOffset` is 3, this.subdomains is `["tobi"]`.

@return {Array}
@api public