support Grunt multitask options

This change allows you to specify options to applied to multiple targets:

```js
umd: {
  options: {
    template: 'path/to/template'
  },
  main: {
    src: 'path/to/src',
    dest: 'path/to/dest'
  },
  alternate: {
    src: 'path/to/src',
    dest: 'path/to/dest'
  }
}
```

This change is backwards-compatible with older versions of `grunt-umd`.  The only caveat is that anything in `options` will overwrite anything in the root `data` object.