v0.6.0: Bump version to 0.6.0

Changes since 0.5.1:

• Drop py2 support.
• Handle long tagline properly.
• Add spell/ and colors/ subdirs to be crawled for doc blocks.
• Fail for duplicate section ids.
• Make @order smarter, not requiring implicit built-in sections to be re-declared and intelligently merging explicit and implicit orders.