add ability to save the output from a command for later reference #15

In a similar vein to the environment variable expansion improvement, it would be nice if here were some global variables that were generated for each command that could then be referenced in subsequent commands. Two example I can think of are:

• The return value / code
• The console output produced

In this way you could do something like this:

> get_current_user
John Doe
> find_user($LAST_OUTPUT)

Or maybe something a bit more language-like, as this example shows:

> result = get_current_user
>find_user($result)