ES6 support

[dy]

Hi @conradz!
Awesome module, could you have a look:

...settings-panel>browserify-graph test.js
...browserify-graph\node_modules\esprima\esprima.js:3859
            throw e;
            ^

Error: Line 11: Unexpected token ILLEGAL
    at throwError (...browserify-graph\node_modules\esprima\esprima.js:1161:21)
    at advance (...browserify-graph\node_modules\esprima\esprima.js:1089:9)
    at lookahead (...browserify-graph\node_modules\esprima\esprima.js:1118:18)
    at match (...browserify-graph\node_modules\esprima\esprima.js:1239:21)
    at parseArguments (...browserify-graph\node_modules\esprima\esprima.js:1569:14)
    at parseLeftHandSideExpressionAllowCall (...browserify-graph\node_modules\esprima\esprima.js:1643:34)
    at parsePostfixExpression (...browserify-graph\node_modules\esprima\esprima.js:1695:20)
    at parseUnaryExpression (...browserify-graph\node_modules\esprima\esprima.js:1730:20)
    at parseMultiplicativeExpression (...browserify-graph\node_modules\esprima\esprima.js:1781:20)
    at parseAdditiveExpression (...browserify-graph\node_modules\esprima\esprima.js:1798:20)

I guess that is because I heavily use ES6.