Footer for Panel

[spyderz42]

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What is the motivation / use case for changing the behavior?
I used a panel to create a "card" to be used in the data grid, so I have my header which gives the name of the card and the body which gives details, but it would be nice to have a footer to put in buttons for things that can be done with the card such as editing. Similar to #41

Please tell us about your environment:

• Angular version: 4.0.X

• PrimeNG version: 2.0.X

• Browser: [all ]

• Language: [all]

• Node (for AoT issues): node --version =