Improve customization

[jfstephe]

Hi, I'd like to be able to say 'format 0.5 as a percent' e.g.

format('###%', 0.5)

and it show as '50%', but I can't see a way to register customizations so ATM I have to do this outside of this library.

My feature request is more for the ability to add in custom masks, with perhaps a callback function rather than this specific example.

Just a suggestion while the re-write is going on :-)

John