Replicate MATLAB's expression and variable handling more closely

[jtrejo13]

Currently, the following expressions are not allowed by the interpreter:

% SETUP
>>>  x = 10
>>>  y = 5

>>>  x        % throws error
>>>  x + y    % throws error

In the real MATLAB, these expressions are evaluated and do not throw an error.

In the case of >>> x, the value of x is printed.

For >>> x * y, the result is assigned to a dummy variable ans