Alpha16 — Fully immutable nodes

Added

• Made nodes fully immutable @KyleAMathews #1035
• Migration guide @KyleAMathews #1032
• Add no-plugins example @scottyeck #1028
• Add support for "internal" plugins #1010
• Expose internal Gatsby data through GraphQL @KyleAMathews #1014

Changed

• Removed updateNode action creator as part of making nodes immutable in #1035.
  Now sites/plugins should use addFieldToNode for adding fields to nodes created
  by other plugins and addNodeToParent for adding a new node as a child to
  an existing node.

Fixed

• Don't override the default onClick handler in gatsby-link @scottyeck #1019