Enable creating custom meta properties

I introduced four new attributes.
- `twitter_site` to add site property to twitter properties like `<meta name="twitter:site" content="@foo">`
- `locale` to add locale property to facebook properties like `<meta property="og:locale" content="en_US">`
- `meta_extras` to add optional properties like `<meta name="foo" content="bar">`
- `meta_custom_extras` to add optional custom properties like `<meta some_key="foo" content="bar">`

@foxbunny Can you review this? Thank you.