readonly support for MultiSelect

[EleanorHughes]

I'm submitting a ... (check one with "x")

[x ] bug report => Search github for a similar issue or PR before submitting
[x ] feature request => Please check if request is not on the roadmap already https://github.com/primefaces/primeng/wiki/Roadmap
[ ] support request => Please do not submit support request here, instead see http://forum.primefaces.org/viewforum.php?f=35

Plunkr Case (Bug Reports)
Plunkr shows p-dropdown read only functionality (working) and p-multiselect read only functionality (not working)
http://plnkr.co/edit/9XrAI2GCT6TTpbLlCfM0?p=preview

Current behavior
When readonly is set to true the p-multiselect is not read only.
Also the disabled property is not what I require.

Expected behavior
With readonly="true" set on the control it should be read only.

Minimal reproduction of the problem with instructions
<p-multiSelect readonly="true" [options]="cars" optionLabel="label">
as per plunkr

• Angular version: 5.2.3

• PrimeNG version: 5.2.2

• Browser:
  Chrome

• Language:
  Typescript 2.4.2

[Raviowitch]

Hello,

Like Eleanor, i need a readonly attribute.
Someone have a workaround ?

Thank you,

Mathieu