`flip` transformation

[miniBill]

I think the package could use a flipHorizontally/flipVertically pair of functions.

My use case is flipping an image (otherwise I'd need two versions of the same image, one normal and one flipped).

[miniBill]

I hereby release the contents of the commit miniBill@aa89422 under the license CC0, or at your choice, MIT

[justgook]

i would suggest instead of flip - add scaleX and scaleY - that can solve that problem, and add more functionality, and flipHorizontally - would be just alias to scaleX -1