Favor key bindings with the most specific CSS selectors

Now if a keypress event bubbles to an element with bindings for
multiple matching selectors, the most specific selector is chosen. This
allows us to override a keybinding by using a more specific selector.
For example, say we have an editor instance inside of the file-finder
widget, so that the user can use all their normal bindings when they
type the name of the file. Except when they hit <esc> we want to close
the file finder, not do whatever they normally have it bound to. Now we
can just say:

```
.file-finder .editor {
  <esc>: close-file-finder
}
```

And we're assured that our binding will take precedence.

src/atom/binding-set.coffee

@@ -1,4 +1,5 @@
 $ = require 'jquery'
+Specificity = require 'specificity'
 
 module.exports =
 class BindingSet
@@ -13,6 +14,7 @@ class BindingSet
   bindings: null
 
   constructor: (@selector, @bindings) ->
+    @specificity = Specificity(@selector)
 
   commandForEvent: (event) ->
     for pattern, command of @bindings

src/atom/key-event-handler.coffee

@@ -1,5 +1,6 @@
 $ = require 'jquery'
 BindingSet = require 'binding-set'
+Specificity = require 'specificity'
 
 module.exports =
 class KeyEventHandler
@@ -12,12 +13,13 @@ class KeyEventHandler
     @bindingSets.push(new BindingSet(selector, bindings))
 
   handleKeypress: (event) ->
-    currentNode = event.target
+    currentNode = $(event.target)
     while currentNode
-      for bindingSet in @bindingSets
-        if $(currentNode).is(bindingSet.selector)
-          if command = bindingSet.commandForEvent(event)
-            $(event.target).trigger(command)
-            return
-      currentNode = currentNode.parentNode
+      candidateBindingSets = @bindingSets.filter (set) -> currentNode.is(set.selector)
+      candidateBindingSets.sort (a, b) -> b.specificity - a.specificity
+      for bindingSet in candidateBindingSets
+        if command = bindingSet.commandForEvent(event)
+          $(event.target).trigger(command)
+          return
+      currentNode = currentNode.parent()