Improved REPL & command line

This is a relatively minor update on the inside - two bugs were fixed, most importantly the very finnicky #70. From the usability, PoV though, this is quite big.

The REPL now allows executing both statements and expressions. This means that there's no need for prefixing them with return - just 1+2 will work. It will also accept blocks with no results (such as function declarations) without unnecessary error messages.

In the same spirit, the main executable now uses a proper command line argument parsing, and includes a --file switch for loading files from disk for execution prior to going into the interactive mode.