Introduce triple bang syntax to remove triggers

In a snippets file, `snippet!!! foo` will remove the trigger from the
current lookup for the current context. Load order still matters.

This is useful to override a series of triggers with different
descriptions, and turn that into a single trigger, so that you don't
have to choose.

autoload/snipMate.vim

@@ -223,9 +223,12 @@ fun! snipMate#ReadSnippetsFile(file) abort
 		if line[:6] == 'snippet'
 			let inSnip = 1
 			let bang = (line[7] == '!')
-			if bang
+			if bang "!!
 				let bang += line[8] == '!'
 			endif
+			if bang "!!!
+			  let bang += line[9] == '!'
+			endif
 			let trigger = strpart(line, 8 + bang)
 			let name = ''
 			let space = stridx(trigger, ' ') + 1
@@ -341,7 +344,13 @@ endfunction
 
 function! snipMate#SetByPath(dict, trigger, path, snippet, bang, snipversion) abort
 	let d = a:dict
-	if a:bang == 2
+	if a:bang == 3
+	  unlet! d[a:trigger][a:path]
+	  if len(d[a:trigger]) == 0
+	    unlet! d[a:trigger]
+	  endif
+	  return
+	elseif a:bang == 2
 		unlet! d[a:trigger]
 		return
 	elseif !has_key(d, a:trigger) || a:bang == 1

doc/SnipMate.txt

@@ -256,6 +256,9 @@ overridden on a per-snippet basis by defining the snippet with a bang (!): >
 Two bangs will remove the trigger entirely from SnipMate's lookup. In this
 case any snippet text is unused.
 
+Three bangs will remove the trigger from the current scope. In this
+case any snippet text is unused.
+
 Note: Hard tabs in the expansion text are required. When the snippet is
 expanded in the text and 'expandtab' is set, each tab will be replaced with
 spaces based on 'softtabstop' if nonzero or 'shiftwidth' otherwise.