Add support for defining a rel alternate tag

[jonoalderson]

In the same way in which users can define a manual canonical URL tag in their post settings, we should also allow them to manually specify a 'rel alt' tag (<link rel="alternate" href="{URL}" />) on a page/post.

We're not in a position to define/support/output this automatically, either for posts or in other scenarios. However, we should definitely allow users page-level abilities to output it.

[jdevalk]

On a page by page level, this seems so trivial? You'd almost always need to code for something interesting here... My gut feeling is, let's not do this for now.