REPL Multi-line support

The JAPL repl is quite friendly already (surely friendlier than its previous python versions). All expressions evaluated in the top level code are printed to stdout once they're returned, but to make the REPL more usable multi-line support is a must have.

It should be quite easy and should not require big changes in the parser, other than maybe the addition of a method to know if the parser reached an EOF, and a check inside the parser to not report EOF errors under specific conditions. If we are at EOF, then maybe the user might want to type something more and instead of erroring out we store the previous code and ask for more input, until the code parses succesfully or causes other syntactical errors