New format() strings

[SteveMyres]

I like the idea of %u, where it will automagically roll between floating point and exponential notation, but you can't specify the number of decimal places like you can with %f, so I'd like to be able to do that.

I'd also like to have a format string for "engineering notation", which is exponential, but with the exponents are limited to multiples of three, and the mantissa can range from one to three places before the decimal.  Thus, the exponential part is always thousands, millions, billions, etc.