Close #3368 - Supercharge :ALEInfo

Make a series of sweeping changes to make :ALEInfo more useful.

1. Deprecate :ALEInfoToClipboard and support :ALEInfo -clipboard
2. Permit :ALEInfo -clip as a shorthand for :ALEInfo -clipboard
3. Support :ALEInfo -preview to render in the preview window
4. Support :ALEInfo -echo for the classic :ALEInfo mode
5. Change the default mode to 'preview', and make it configurable
6. Add syntax highlighting for ALEInfo in preview mode
7. Add a convenience to look up documentatation that explains itself
8. Don't show an empty 'Linter Variables' section

ftplugin/ale-info.vim

@@ -0,0 +1,16 @@
+" Close the ALEInfo preview window with the q key.
+noremap <buffer> q :q!<CR>
+
+" Explicitly use the default synmaxcol for ale-info.
+setlocal synmaxcol=3000
+
+function! ALEInfoOpenHelp() abort
+    let l:variable = matchstr(getline('.'), '\v[gb]:ale_[a-z0-9_]+')
+
+    if !empty(l:variable)
+        execute('help ' . l:variable)
+    endif
+endfunction
+
+" Press space to open :help for an ALE Variable
+nnoremap <buffer> <silent> <space> :call ALEInfoOpenHelp()<CR>