Add support for more ALE highlight groups?

[dlukes]

seoul256.vim already supports ALEErrorSign and ALEWarningSign. Would it be possible/acceptable to also add the following?

• ALEError
• ALEWarning
• ALEInfo
• ALEInfoSign

In my case, LanguageClient-neovim relies on these groups to highlight diagnostics, so I'm currently overriding them with other groups to get colors and underlines/squigglies when using seoul256. But it might be nice for users who are unaware of this if it worked out of the box.

And thanks for an awesome theme!