Allow array of glob pattern for files option by changing to src

The files option only allows a string for specifying paths, an array would be very helpful.

This works, but will also compress all the SVG graphics in the `node_modules` folder:

```coffee
svgo:
  optimize:
    files: '**/*.svg'
```

Because the `files` option gets automatically parsed by Grunt, I changed it to `src` to make it compatible with an array. I would prefer to use `files` but it doesn't work because this plugin doesn't use `src` and `dest` options.

```coffee
svgo:
  optimize:
    src: ['**/*.svg', '!node_modules/**']
```