Xcode playgrounds preparation script

[twof]

A community member (@gitinsky on discord) detailed the work needed to get Vapor/Fluent up and running in an Xcode playground

https://medium.com/@serzhit/how-to-make-vapor-3-swift-playground-in-xcode-10-c7147b0f7f18

I think the scripts included in the article would make a good subcommand for the toolbox. Something like

vapor prepare-playground

[tanner0101]

@loganwright we should consider adding this to toolbox 2018

[joscdk]

There was much discussion on that in the random channel :D

[tanner0101]

Would be nice to get a summary of the discussion here if possible.