[WIP] Add support for including single Swift files

[Jeehut]

I've just begun implementing the feature suggested in #40. This is a simple implementation and could already be working, I haven't added tests yet. @yonaskolb Feel free to give feedback already though, if you like. I will probably not be able to continue my work for a few days, please stay tuned (or feel free to continue it yourself, if you like).

TODOs:

• Implement single file include support
• Test single file include support
• Make sure multiple specifications of the same dependency in different files are reduced to one
• Document including files, warning against global private or fileprivate access identifiers
• Update beak edit with // FILE: beakScript/Utility.swift comments to separate files
• Document the different beak edit behavior using included files
• (Optional) Add support for including all files in a directory
• (Optional) Test inclusion of entire directory (making sure only Swift files are added)