Add percentage-equal-column Fill Mode

[ddrocco]

Hi, I have need for an ember table which uses an equal-column fill mode on a percentage basis, e.g.:

['100px', '100px', '200px'] and inserting a new element with width ['100px'] for a table which is exactly 400px should yield these sizes:
['80px', '80px', '160px', '80px']

Currently, it'll pull equally from all regardless of size, yielding:
['66px', '67px', '167px', '100px'] or ['75px', '75px', '175px', '75px'] (not sure on the exact addition logic - it's pretty messy for my purposes.