Support fragment navigation in TOC by emitting `id` attributes for each href

This first [came up](https://gitter.im/sbt/sbt?at=5696ddfc2bc35f6c1c19f29d) on the SBT gitter channel. @eed3si9n mentioned that the HowTo section might be the right place to put some docs, and incidentally I noticed some bad UX if you just want to view all the HowTo articles - you have to scroll down for like 10 screen to get to them - as can be seen here:
http://www.scala-sbt.org/0.13/docs/Howto.html

Because the SBT whole TOC is huge, it would be nice to be able to fragment link to a specific place within the TOC. That would be enabled if pamflet converted & emitted each `page.name` into an `id` for each `href`, about here I think: https://github.com/n8han/pamflet/blob/29eb5483b5bdd56c8fb38e55b6c853c862439352/library/src/main/scala/printer.scala#L39