ShellOut

This release enables a custom Process instance to be injected when calling shellOut.

• A race condition that could occur when capturing output data is now being avoided (by @helje5)
• ShellOut no longer closes standard file handles (like stdin and stderr) when finishing (by @SteveBarnegren)

This version migrates ShellOut to Swift 4.2

ShellOut is now thread safe thanks to @harlanhaskins 🎉

• ShellOut now uses Swift 4.
• copyFile and removeFile has been added to the convenience API.
• You can now use paths containing a tilde to refer to the home folder.
• More user friendly descriptions of errors.