Add option to ignore asm.js code

I'm wanting to minify a project which contains some Emscripten generated asm.js. As babili cannot safely minimise it and Emscripten already minimises the asm.js to a large extent, an option to ignore all code in an asm.js function would be very helpful.

Future work could selectively enable or disable transformations which are safe for asm.js, but for now a global switch would be great.