syntax: Prevent syntax/html.vim from overriding our syn sync

When including other syntax types it's necessary to declare that we're
the main syntax, otherwise they will apply their own syn sync settings.

Here, by including html.vim in 7e00da6,
we end up with syn sync minlines=10 maxlines=100 and we sync using
html.vim syncing rules, which isn't at all what we want. Try opening
large_file.ex in the middle, syntax will be incorrect.

Author: liskin

syntax/elixir.vim

@@ -1,5 +1,8 @@
-if exists("b:current_syntax")
-  finish
+if !exists("main_syntax")
+  if exists("b:current_syntax")
+    finish
+  endif
+  let main_syntax = "elixir"
 endif
 
 let s:cpo_save = &cpo
@@ -230,5 +233,9 @@ hi def link elixirPrivateRecordDeclaration   elixirRecordDeclaration
 
 let b:current_syntax = "elixir"
 
+if main_syntax == "elixir"
+  unlet main_syntax
+endif
+
 let &cpo = s:cpo_save
 unlet s:cpo_save