Multiline pipeline indentation

This seems to be the most common style: it's used in the docs and in big projects like Phoenix.

See replies to https://twitter.com/henrik/status/529731017511559168.

It does seem like some people do it differently. Not sure how prescriptive you want to get in this style guide, and what level of consensus you want before you include something. I know I wondered about the idiomatic style for this and couldn't find it stated explicitly anywhere, hence this pull request.