[Feature request] Add disabled option for mdl-option

[thaihacong]

Hi @mseemann, currently, the module only support disabled option for mdl-select.
Could you please add disabled option for mdl-option also?

For now, I am using workaround by adding a .disabled css class to mdl-option as follow,

mdl-select {
  mdl-option.disabled {
    pointer-events: none;
    cursor: default;
  }
}

However, when I click on the disabled option, mdl-popover is still closed.