Release v0.2.1

[tyru]

• Go 1.9.1 or higher with the patch for os.RemoveAll() is installed (ref [WSL] 'volt rm tyru/caw.vim' always fail at the first run #1)
  • go version is 1.9.1 or higher
  • On WSL
    • If tyru/caw.vim exists, run bin/volt rm -r tyru/caw.vim
    • Check if bin/volt get tyru/caw.vim && bin/volt rm -r tyru/caw.vim will succeed
• Merge these changes into master
  • Merge devel
  • Bump version number of voltVersion variable in cmd/version.go
  • Add new features to README.md
    • TODO: can edit README.md on non-master branch from GitHub?
• Write release notes (merged PR)
• Push release tag
• Upload 6 binaries: {linux,windows,darwin}_{386,amd64}

[tyru]

Done.
https://github.com/vim-volt/volt/releases/tag/v0.2.1