Pretty printing

[tanhevg]

Thanks for the lovely package!

One feature that I miss from JSON2.jl is pretty printing. One quick way of providing this would be just lifting pretty.jl from there into this package. Ideally it would probably be neater from the users' point of view to have a keyword argument in write(..., pretty=true), such that we don't need to blow up the string in memory for large structs.

[Eric Forgy]

To do this, we'll need to port over

https://github.com/quinnj/JSON2.jl/blob/master/src/pretty.jl

(note to self) :)

[Jacob Quinn]

Implemented in #67