Add autodiscovery of feed.xml to layout.erb

Some users forget to add a link or something to `feed.xml` and visitors to their blog can't notice the feed.
This change would be helpful for them.

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The Travis CI build failed because of `cannot load such file -- middleman-more/extensions/automatic_alt_tags`. I think this is related to the problem mentioned in [Pull Request #1072 in middleman/middleman](https://github.com/middleman/middleman/pull/1072), not to this change.