Add extractSourceLocation option

This option will provide metadata in the Message Description for the
file and location in the file where the message was defined. When
enabled and running it on [this file][1] you'll get this result:

```json
[
  {
    "id": "app.title",
    "defaultMessage": "React Intl Translations Example",
    "file": "src/client/components/app.js",
    "start": {
      "line": 23,
      "column": 20
    },
    "end": {
      "line": 26,
      "column": 22
    }
  },
  {
    "id": "app.locales_menu_heading",
    "defaultMessage": "Locales:",
    "file": "src/client/components/app.js",
    "start": {
      "line": 32,
      "column": 20
    },
    "end": {
      "line": 35,
      "column": 22
    }
  }
]
```

Fixes #71

[1]: https://github.com/yahoo/react-intl/blob/25f3f64d86ac86281157117b5bdb9fa30d1d615f/examples/translations/src/client/components/app.js

Author: ericf

src/index.js

@@ -107,7 +107,7 @@ export default function () {
     }
 
     function storeMessage({id, description, defaultMessage}, path, state) {
-        const {opts, reactIntl} = state;
+        const {file, opts, reactIntl} = state;
 
         if (!(id && defaultMessage)) {
             throw path.buildCodeFrameError(
@@ -134,7 +134,15 @@ export default function () {
             );
         }
 
-        reactIntl.messages.set(id, {id, description, defaultMessage});
+        let loc;
+        if (opts.extractSourceLocation) {
+            loc = {
+                file: p.relative(process.cwd(), file.opts.filename),
+                ...path.node.loc,
+            };
+        }
+
+        reactIntl.messages.set(id, {id, description, defaultMessage, ...loc});
     }
 
     function referencesImport(path, mod, importedNames) {
@@ -257,7 +265,7 @@ export default function () {
 
                     // Evaluate the Message Descriptor values, then store it.
                     descriptor = evaluateMessageDescriptor(descriptor);
-                    storeMessage(descriptor, path, state);
+                    storeMessage(descriptor, messageObj, state);
                 }
 
                 if (referencesImport(callee, moduleSourceName, FUNCTION_NAMES)) {