Allow customization of cardCellMargins for individual CardsViewControllers

[rcole34]

add field in CardsViewController to allow customization of cardCellMargins

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[badrinathvm]

@rcole34 Looks Good to me , Can we test this change against hosting app's that it wont break the layouts ?

[rcole34]

@badrinathvm I did some manual testing and everything seems to be fine, as it still uses the CardParts.theme.cardCellMargins as the default.

[rcole34]

@croossin I double checked in hosting apps, and can't see any problems with broken margins. I added the documentation for it under CardsViewController, as that is where the changes are made, and tried to follow the same nested pattern, but let me know if you want me to change anything with that!

[croossin]

Looks great! Will get out in next release.