Add 'mainRequireConfigFile' option to make it possible to extract config from a file (similar to r.js)

This builds on the idea of [#10](https://github.com/jsoverson/grunt-template-jasmine-requirejs/pull/10) to let the user include a require configuration from an external file.

This pull request includes tools from [r.js](https://github.com/jrburke/r.js) (the tools are copied from r.js instead of included as dependencies - see last comments on https://github.com/jrburke/r.js/issues/385). These tools are used to extract the config from `options.mainRequireConfigFile` and merge it with `options.requireConfig` (note that the latter overrides the former).