Add variable to control AutoTrigger

Now it is possible to disable AutoTrigger via global variable `g:UltiSnipsAutoTrigger`,
and to dynamically toggle AutoTrigger on/off with `Ultisnips#ToggleAutoTrigger()`.

autoload/UltiSnips.vim

@@ -123,6 +123,11 @@ function! UltiSnips#CanJumpBackwards() abort
 	return can_jump_backwards
 endfunction
 
+function! UltiSnips#ToggleAutoTrigger() abort
+    py3 vim.command("let autotrigger = %d" % UltiSnips_Manager._toggle_autotrigger())
+    return autotrigger
+endfunction
+
 function! UltiSnips#SaveLastVisualSelection() range abort
     py3 UltiSnips_Manager._save_last_visual_selection()
     return ""

pythonx/UltiSnips/snippet_manager.py

@@ -146,6 +146,10 @@ def __init__(self, expand_trigger, forward_trigger, backward_trigger):
         if enable_snipmate == "1":
             self.register_snippet_source("snipmate_files", SnipMateFileSource())
 
+        self._autotrigger = True
+        if vim_helper.eval("exists('g:UltiSnipsAutoTrigger')") == "1":
+            self._autotrigger = vim_helper.eval("g:UltiSnipsAutoTrigger") == "1"
+
         self._should_update_textobjects = False
         self._should_reset_visual = False
 
@@ -852,6 +856,10 @@ def can_jump_forwards(self):
     def can_jump_backwards(self):
         return self.can_jump(JumpDirection.BACKWARD)
 
+    def _toggle_autotrigger(self):
+        self._autotrigger = not self._autotrigger
+        return self._autotrigger
+
     @property
     def _current_snippet(self):
         """The current snippet or None."""
@@ -956,7 +964,8 @@ def _track_change(self):
                 before = vim_helper.buf.line_till_cursor
 
                 if (
-                    before
+                    self._autotrigger
+                    and before
                     and self._last_change[0] != ""
                     and before[-1] == self._last_change[0]
                 ):