[refactored] rewrite to have options
Not sure what the right pattern is for operator APIs. Some (like rewriteTo) will likely never need options, so requiring every operator to use the named args pattern would be noisey, but consistent. Passing an options dict as the last arg is messier, but avoids cluttering the APIs that don't need options.
For now, I care more about functionality than API design, so I'm using an options dict.
Tags: Material Motion
Differential Revision: http://codereview.cc/D3315