Ale: Support neovim floatwindow

[glepnir]

Coc.nvim can show the lsp linter message in the float window。can ale support this feature？

[gbrlsnchs]

There are three things I think would be nice to show in a floating window:

• Hover information
• Documentation while navigating the items in popup menu
• Linting info

[pianocomposer321]

Any update on this? I would really like to see floating windows implemented.

[rbhanot4739]

I would also love to have this feature implemented as it makes vieweing linter messages much user friendly than having them displayed through echo or virtualtext.