support notifications for window/showMessage

[teto]

Would be nice to show notifications for "window/showMessage". For instance when having this notification

[ DEBUG ] 2020-10-13T22:28:38+0200 ] ...zj1z0zz-neovim-master/share/nvim/runtime/lua/vim/lsp.lua:434 ]	"notification"	"window/showMessage"	{  message = "No [cradle](https://github.com/mpickering/hie-bios#hie-bios) found for /home/teto/.cache/bazel/_bazel_teto/32070fb26dd3cfe8bd075800f8bb83d1/execroot/toplevel_workspace/bazel-out/k8-fastbuild//HspecDriver_unit.hs.\n Proceeding with [implicit cradle](https://hackage.haskell.org/package/implicit-hie)",  type = 2}

I only see the end of the message in the echo area Proceeding with [implicit cradle](https://hackage.haskell.org/package/implicit-hie) instead of the whole informative message.
I've started hacking some local fix with https://github.com/nvim-lua/popup.nvim, also I would like neovim to have a "notification" provider but meanwhile it could be a cool option for diagnostic-nvim.