Add the hreflang="x-default" meta tag

Some details about the new tag: http://googlewebmastercentral.blogspot.com/2013/04/x-default-hreflang-fo...

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Question so, does metatags module already support hreflang?
Otherwise I could have a look for this specific x-default feature but in this module: i18n contrib because we are already supporting basic hreflang there.

Given that I'm recommending using the hreflang module instead, this issue is being handled by another issue: #2376417: Add x-default

It turns out that the hreflang module doesn't encompass all of the functionality needed, so this is being reopened.

This is being handled by #2597301: Expand hreflang support, auto-add fields for each locale.