Bind meta-S to save as in editor

Close #144

src/app/keymaps/atom.cson

@@ -22,7 +22,7 @@
   'pageup': 'core:page-up'
   'pagedown': 'core:page-down'
 
-  'meta-S': 'window:save-all'
+  'meta-alt-s': 'window:save-all'
   'meta-W': 'window:close'
   'meta-+': 'window:increase-font-size'
   'meta--': 'window:decrease-font-size'
@@ -33,4 +33,4 @@
 '.tool-panel':
   'meta-escape': 'tool-panel:unfocus'
   'escape': 'core:close'
-  'meta-w': 'noop'
+  'meta-w': 'noop'

src/app/keymaps/editor.cson

@@ -1,5 +1,6 @@
 '.editor':
   'meta-s': 'editor:save'
+  'meta-S': 'editor:save-as'
   'enter': 'editor:newline'
   'meta-enter': 'editor:newline-below'
   'tab': 'editor:indent'