FR: add "amount" attr

[pixlpa]

I'm using ease on a project right now and the client wants to be able to adjust the curves, so I've been lerp-ing the output of ease objects with a linear value. Would be nice to add an "amount" attr to ease objects to do that internally like:
output = (eased*amount)+(linear*(1-amount));

[tap]

Basically a "mix" or wet/dry. What if you wanted to put an ease function on that too? ;-)