Support multi-line input in REPL mode

[danilhendrasr]

Currently yalija's REPL mode only supports single-line statements and expressions, this is a quite jarring limitation because practically no one writes code in a single line. Moreover, PR #9 ships the support for lexical scoping, it would be weird if the only way to use or test the feature in REPL mode is by cramming the code into a single line.