Added support for "itemprop" in custom hashes

[kpumuk]

I am not sure how I feel about this change yet. Basically, it starts treating itemprop attribute in a special way, applying itemprop attribute to _ tag:

display_meta_tags(
  og: {
    image: {
       _:       'image.png',
      itemprop: 'image',
      type:     'image/jpeg',
      width:    200,
      height:   {
        _:        200,
        itemprop: 'custom',
      },
    },
  },
)

Closes #198

[kpumuk]

Merged in #227