Add `--version` option

[guidanoli]

LSP clients like NeoVim plugin nvim-lspconfig display LSP versions on :LspInfo.
It tries several options: version, -version, --version and even --help.
However, yaml-language-server supports none of those options.
So, on diagnostics, it is displayed like such:

  version:           `?` (Failed to get version) Tried:
  `.../yaml-language-server --version`
  `.../yaml-language-server -version`
  `.../yaml-language-server version`
  `.../yaml-language-server --help`