Lets us define default animation options by animation family that can be overridden by the user at runtime. Demonstration of usage with animate_gather() and animate_spread()

And demonstrates usage with the tidyr gather/spread animators. In essence, user-facing functions take an argument `anim_opts = anim_options()` as a last argument (after ... if applicable). These options should be passed through to lower level functions and should be given to `animate_plot()` and `static_plot()`.

This implementation actually doesn't break previous functions, but the join and set families will need to be updated to match.

And change Long/Wide to lowercase to reflect that they are both variables names and a description of the state of the data.

-id1, -id2 work, id1:id2 does not (yet)