Allow for the use of a "paths" option

This will let you create shortcuts to paths in your Gruntfile.

For example, if you want to include some Javascript from a node module,
specifying the paths option like this:

```
import: {
  options: {
    paths: {
      some_module: 'node_modules/some_module/path/to/js'
    }
  },
  build: {
    src: 'src/js/main.js',
    dest: 'dist/js/main.js',
  },
}
```

This will let you reference Javascript files from `main.js` like this:

```
@import "some_module/foo.js"
@import "some_module/bar.js"
@import "some_module/baz.js"
```