Fix: support for consumers of the REPL *module* being able to opt into using the global context ...

... via option `.useGlobal`.

Note that, at least for now, CoffeeScript's own REPL *CLI* still uses a
non-global context, rendering modules such as `color`, which attempt to
modify the prototypes of JavaScript primitives, ineffective. By
contrast, node's own CLI does use the global context.