Version 0.4.0

• Add diffmode plugin interface and dynamic library loading (#156 )
  • Add bundled numeric diff plugins: numeric-diff and numeric-inline-diff (#67 )
  • Add sample cdylib plugin for custom diffmode development
  • Add --diff-plugin option and plugin examples/documentation
• Add -g / --chgexit option to exit when output changes
• Add --ignore-spaceblock option for whitespace block-insensitive diff
  • Support built-in diff modes and bundled plugins
• Improve history display to follow the selected output mode
• Refactor internal crates and diffmode implementation