Customizing `REPL_PROMPT`

[Zearin]

So, repl checks for a REPL_PROMPT environment variable on startup.

I tried using the environment variable to colorize the prompt, but repl prints the color escape sequences literally. I also can’t use #{ARGV[0]} because it, too, is printed out verbatim.

These two things make customization with REPL_PROMPT pretty useless.

[lukateras]

Duplicate of issue #16.