Customize grunt-swig with additional tags.

I hope to have make this as customizable as possible.

What you can achieve is:

Have a project called XYZ that requires `grunt-swig` AND `swig-sometag` (as siblings).  
**XYZ\Gruntfile.js**:
```javascript
grunt.initConfig({
  swig: {
    documentation: {
      dest: 'test/dest',
      src: ['**/*.swig', '!templates/*.swig'],
      tags: {
        sometag: require('swig-sometag')
      }
    }
  }
});
```

Or you could use a custom swig-tag made by yourself! Just using:

```javascript
grunt.initConfig({
  swig: {
    documentation: {
      dest: 'test/dest',
      src: ['**/*.swig', '!templates/*.swig'],
      tags: {
        personal: require('./my-personal-tag')
      }
    }
  }
});
```