Add support for ltex-ls-plus

[120ace]

Ltex-ls-plus (https://github.com/ltex-plus/ltex-ls-plus) is a maintained fork of ltex-ls. It is currently 166 commits ahead of ltex-ls itself. Recently, just a week ago (Dec 7), nvim-lspconfig even added a preset for it (https://github.com/neovim/nvim-lspconfig/blob/master/lua/lspconfig/configs/ltex_plus.lua)!

It would be great if your amazing plugin could (optionally) use this maintained fork of ltex-ls as it's lsp backend.

[michix]

What I like a lot is that ltext_plus also supports AsciiDoc.