Provide opts object to resolve() with single possible key — extensions

Underlying *resolve* package allows resolving modules against non-js files. I think this is important feature to also support `browser` field for those non-js files — it will allow to use coffeescript (and others like typescript or roy) with *module-deps* and then with *browserify* w/o requiring to specify extensions in `require(...)` calls which is much saner.

Regarding test cases — for now, I simply copied them from `test/module.js` and `test/local.js` and `test/fixtures` to `test/module-coffee.js` and `test/local-coffee.js` and `test/fixtures-coffee` correspondingly.