v0.21.0

• Fix hasattr for data.values when key is not there (issue #56)
• [experimental] Enhance strict mode to catch missing newline before document start
  • example of such mistaken YAML observed in the wild

05bc4510c3212d3926bf431456fdbeedf635158d4e0e225ec028dce71ff638d7  ./ytt-darwin-amd64
88c755c31a4d85d0bdb7a32e7ee34a93c45222a14f4f6204ce65e0da8875452f  ./ytt-linux-amd64
6197c5a0c19c96c3252b1dc548e2add4a6b52fb0340c784143c4e1749a7199c5  ./ytt-windows-amd64.exe

(Installation instructions: https://k14s.io/#install-from-github-release)