Chokes on ES6 modules.

Currently, running module-deps on source with ES6 module syntax throws `Error: Parsing file /blah/dee/dah.js 'import' and 'export' may appear only with 'sourceType: module' (4:0)`.

The reason for this is that `acorn`, via `detective`, needs a `{ecmaVersion: 6, sourceType: 'module'}`, but [currently](https://github.com/substack/module-deps/blob/master/index.js#L433), there's no way to pass those options through.
#63 would address this, although it would only be a superficial fix: module-deps still wouldn't actually traverse ES6 modules. I'm assuming ES6 modules are out of scope here?

Also: anything I could do to help get #63 merged?