Forced strict mode

ES6 module code is always strict (see [10.2.1](http://www.ecma-international.org/ecma-262/6.0/index.html#sec-strict-mode-code)), and `much` passes `sourceType: 'module'` setting to acorn.

This leads to errors when the code is taking advantage of non-strict features:

``` python
$ much v8-earley-boyer.js
SyntaxError: Octal literal in strict mode (1076:5)
```
