modules folder name is configurable

Make the name of the modules folder configurable to something other than `node_modules` by setting `opts.modules`.
This allows you to use the node resolve algorithm to resolve all kinds of crazy stuff like config files, templates etc.

``` js
var resolve = require('resolve');
// use config instead of node_modules and find the nearest config file
var configPath = resolve('test', {
    modules: 'config',
    extensions: ['json']
});
// use views for our templates
var templatePath = resolve('my-template', {
    modules: 'views',
    extensions: ['html']
});
```