Add support for ' (thousands separator) for Printf

[oheil]

There was a thread recently:

Is it possible for Julia to print numeric output in the REPL with underscores as digit separators?

Say I need to compute the size of a cross join before attempting it. My eyes aren’t great. julia> 1_000*144*30*20*15*18 23328000000 Wait, how many 0s are there? julia> ans==2_332_800_000 false Urgh. Can I just see the output with the _ in the first place? Then I could also nicely copy and paste it into future computations.

It’s not about printf so maybe you like it anyways.