allow plugins to override the default mappings

Author: scrooloose

doc/NERD_tree.txt

@@ -1060,6 +1060,9 @@ NERDTreeAddKeyMap({options})                             *NERDTreeAddKeyMap()*
     "callback" - the function the new mapping will be bound to
     "quickhelpText" - the text that will appear in the quickhelp (see
     |NERDTree-?|)
+    "override" - if 1 then this new mapping will override whatever previous
+    mapping was defined for the key/scope combo. Useful for overriding the
+    default mappings.
 
     Additionally, a "scope" argument may be supplied. This constrains the
     mapping so that it is only activated if the cursor is on a certain object.

lib/nerdtree/key_map.vim

@@ -134,8 +134,14 @@ endfunction
 
 "FUNCTION: KeyMap.Create(options) {{{1
 function! s:KeyMap.Create(options)
-    let newKeyMap = copy(self)
     let opts = extend({'scope': 'all', 'quickhelpText': ''}, copy(a:options))
+
+    "dont override other mappings unless the 'override' option is given
+    if get(opts, 'override', 0) == 0 && !empty(s:KeyMap.FindFor(opts['key'], opts['scope']))
+        return
+    end
+
+    let newKeyMap = copy(self)
     let newKeyMap.key = opts['key']
     let newKeyMap.quickhelpText = opts['quickhelpText']
     let newKeyMap.callback = opts['callback']