Add grow to xy-cell-base
Add grow to xy-cell-base

Added the ability to use the xy-cell-base mixin to grow the flex item to take up all available space.

This is effectively the same as: `.flex-child-grow`, found @ https://foundation.zurb.com/sites/docs/flexbox-utilities.html#vanilla-flexbox-helper-classes - But there isn't a way to do this through the SASS mixins.
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