Implement Lua ALE setup & overhaul documentation

1. Add ale.setup and ale.setup.buffer for pure Lua configuration.
2. Update many global settings to use Booleans instead of numbers to
   make types easiert to work with in Lua.
3. Radically reformat documentation and fix errors to make
   documentation more usable for Neovim users.

test/completion/test_lsp_completion_messages.vader

@@ -1,11 +1,14 @@
 Before:
   Save g:ale_completion_delay
+  Save g:ale_completion_enabled
   Save g:ale_completion_max_suggestions
   Save g:ale_completion_info
+  Save g:ale_completion_autoimport
   Save &l:omnifunc
   Save &l:completeopt
 
-  let g:ale_completion_enabled = 1
+  let g:ale_completion_enabled = v:true
+  let g:ale_completion_autoimport = v:true
 
   call ale#test#SetDirectory('/testplugin/test/completion')
   call ale#test#SetFilename('dummy.txt')