Process settings once in SettingGraph for better performance.

Running 'inspect tree sbt/update' on sbt goes from 27 s to <1 s.
Review by @eed3si9n.
updated links for my plugins that moved to the sbt org on github, added testng plugin
Docs: fix taskKey usage in Custom Settings example

Following commentary on 126f41c0ac1
Merge branch '0.13' into fix_split_merge