Add PrettyTables rendering

[jpsamaroo]

As discussed at JuliaCon, it would be great to use PrettyTables to render the DTable via the show method.

[krynju]

reverted https://github.com/JuliaParallel/DTables.jl/pull/45 performance was weird and i was getting hangups locally

we need to reevaluate how to implement this correctly and possibly make it opt in instead of active by default