Set max depth

[alexflint]

I use this package regularly across several projects - thanks for building it!

Would it be possible to add a way to control the max depth? At the moment I see it is set to 10 but sometimes I want more:

https://github.com/kr/pretty/blob/master/formatter.go#L95

[Heanthor]

I'd like to second this issue, but for another reason. I have a simple struct that happens to contain a time.Time, and this library prints out hundreds of lines of internal time info.