Add demo for expandable row groups

[yasin-mansour]

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

Current behavior There is no expandable row groups support in the new turbo Table

Expected behavior Since DataTable marked as deprecated in the last release, I would like to have a support for expandable row groups in the turbo Table, so I can easily migrate from DataTable to turbo Table.

Please tell us about your environment:

• Angular version: 5.0.0

• PrimeNG version: 5.2.0

• Browser: [all | Chrome XX | Firefox XX | IE XX | Safari XX | Mobile Chrome XX | Android X.X Web Browser | iOS XX Safari | iOS XX UIWebView | iOS XX WKWebView ]

• Language: [all | TypeScript X.X | ES6/7 | ES5]

[ivijan]

+1 for implementation of expandable row groups!