pretty-printing of AST dumps and a bit of configurability for REPL

    moe> 1 + 2
    {"CompilationUnitNode" : {"ScopeNode" : {"StatementsNode" : [{"BinaryOpNode" : {"lhs" : {"IntLiteralNode" : 1}, "operator" : "+", "rhs" : {"IntLiteralNode" : 2}}}]}}}
    3

Turn on pretty-printing (use **on**, or **yes**, or **1** for option values)

    moe> :set prettyPrintAST 1

    moe> 1 + 2
    {
      "CompilationUnitNode" : {
        "ScopeNode" : {
          "StatementsNode" : [
            {
              "BinaryOpNode" : {
                "lhs" : {
                  "IntLiteralNode" : 1
                },
                "operator" : "+",
                "rhs" : {
                  "IntLiteralNode" : 2
                }
              }
            }
          ]
        }
      }
    }
    3

Other options handled for now: **printOutput** and **dumpAST**