Add ColorFn to imgui_memory_editor.h

I added ColorFn locally a long time ago and I find it very useful for highlighting changed values in emulator debug memory views.

I think this would be a useful addition to the repo, but there are a few issues I am aware of:

1. I'm not sure whether ColorFn or OptGreyOutZeroes should have priority. I think ColorFn because it can implement OptGreyOutZeroes internally.
2. ColorFn is not used with OptShowHexII (I don't understand the use case here)
3. I'm wary of conversions of user provided ImU32 to Float4 for each byte for TextColored