Add REPL

CLI option is --repl, shortcut is -r
Split FancyScript.compile() into .parse(), .compile(), .js()
.parse() is FancyScript source -> FancyScript AST
.compile() is FancyScript AST -> JS AST
.js() is JS AST -> JS source
This follows CoffeeScript's model
Document recent changes in README
0.5.0