Feature request: Custom formatting

[svenstaro]

Hey, I have the need to format for fairly specific formatting. It'd be great if the library had custom formatting built-in. It would also already be a help if I could change the number of decimals.

It would also be convenient if I could choose a specific unit to output this as. Currently I can only to .as_u64() but ideally I could do .as_mb() or similar and then receive a u64 that's actually megabytes.