Support playgrounds with external framework dependencies

[interstateone]

I think this is normally done automatically by Xcode in the case where a playground is part of a workspace. It does something like adds the build products directory to the frameworks search path. Not sure what the UI for this should be, because JungleGym doesn't have support for opening an entire xcworkspace.

Ideas:

• Allow opening a xcworkspace. Maybe only show the playgrounds inside but detect the build products directory
• Allow selecting a workspace to detect the build products directory but not for opening. The playground would still be the opened document.
• Allow specifying a build products path
• Allow specifying a list of built frameworks