Added custom installation mode
Added custom installation mode

Start with parameter "custom" to set ports manually

alexanderjackson/seafile-server-installer,felag/seafile-server-installer,haiwen/seafile-server-installer,seafile/seafile-server-installer,SeafileDE/seafile-server-installer,syncwerk/syncwerk-server-installer,shoeper/seafile-server-installer,necrevistonnezr/seafile-server-installer