Add ECMAScript version filter

[nzakas]

This feature allows an extra ecmascript option to be passed to the parse() method. This determines the syntax will be accepted by the parser. The default is all syntax (similar to Esprima) and the valid values are currently 5 and 6.

To test, it's easiest to use let and const. Espree already parses them but technically these are not allowed in ECMAScript 5. So when ecmascript is 5, using let and const should throw an error.