This lets you return a plain JS string (like before), but pass the wrapOutput option to common.register() in order to wrap this string in a ShellString. This wraps it with the following default parameters (which I think will work properly most of the time):

return myReturnValue;
// This gets wrapped to imitate the statement:
return new common.ShellString(myReturnValue, common.state.error, common.state.errorCode);
// If any errors have occurred, put these in the .stderr attribute
// Whatever the resulting exit code is (0 unless there's an error), set that for the .code attribute

By default, common.register() works as before (so collaborators must opt-in for this feature). This lets functions return non-string output, or return a custom-built ShellString (if these default values aren't suitable).

Do not merge until after #479 is either closed or merged