undo 0.27.0

An undo-redo library with dynamic dispatch and automatic command merging.
Documentation
environment:
    matrix:
      - TARGET: x86_64-pc-windows-msvc
      - TARGET: i686-pc-windows-msvc
      - TARGET: x86_64-pc-windows-gnu
      - TARGET: i686-pc-windows-gnu
install:
    - curl -sSf -o rustup-init.exe https://win.rustup.rs/
    - rustup-init.exe -y --default-host %TARGET%
    - set PATH=%PATH%;C:\Users\appveyor\.cargo\bin
    - rustc -vV
    - cargo -vV
build: false
test_script:
    - cargo build --verbose
    - cargo test --verbose