Before this commit, if user does this:
let g:node_host_prog = '~/.nvm/versions/node/v11.3.0/bin/neovim-node-host'
the "~/" is not expanded to user's home directory.
`:help g:ruby_host_prog` suggests a path with "~/" so technically we
already claimed to support this.
closes https://github.com/neovim/node-client/issues/102