Add REPL tab auto-completion

Adds auto-completion options on REPL.

Problem to solve

When user presses TAB, currently the REPL gives no support for auto-completion.

Proposal

The REPL console could search for available entries in the global scope, or even local scope of a method declaration, and attempt to propose meaningful suggestion for text auto-completion when TAB is pressed.

Reference:

• https://thoughtbot.com/blog/tab-completion-in-gnu-readline