Support cross-file quickfix/location list

Like LanguageClient-neovim does. Taking Rust as an example, the entire output of an RLS run on a workspace is put into the quickfix list, and every buffer always has diagnostics immediately available after a single run, and `:cn` and `:cN` (equivalent of `:ALENext` and `:ALEPrevious`) work across all files. (I‘d suggest to keep offering the wrapping complements which for the quickfix list in general can be had with `vim-qf`.)