Adds configuration option for large file warning threshold.

src/config-schema.coffee

@@ -122,6 +122,10 @@ module.exports =
           {value: 'no', description: 'Do not send any telemetry data'}
           {value: 'undecided', description: 'Undecided (Atom will ask again next time it is launched)'}
         ]
+      warnOnLargeFileLimit:
+        description: 'Warn before opening files larger than this number of megabytes.'
+        type: 'number'
+        default: 20
   editor:
     type: 'object'
     properties:

src/workspace.coffee

@@ -550,7 +550,7 @@ class Workspace extends Model
     fileSize = fs.getSizeSync(filePath)
 
     largeFileMode = fileSize >= 2 * 1048576 # 2MB
-    if fileSize >= 20 * 1048576 # 20MB
+    if fileSize >= @config.get('core.warnOnLargeFileLimit') * 1048576 # 20MB by default
       choice = @applicationDelegate.confirm
         message: 'Atom will be unresponsive during the loading of very large files.'
         detailedMessage: "Do you still want to load this file?"