Add warning when using <FormattedPlural>

This adds a warning when someone is using this Babel plugin and they
are using <FormattedPlural> instead of <FormattedMessage> because the
default messages will not be extracted.

Fixes #11

Author: ericf

src/index.js

@@ -12,6 +12,7 @@ import printICUMessage from './print-icu-message';
 const COMPONENT_NAMES = [
     'FormattedMessage',
     'FormattedHTMLMessage',
+    'FormattedPlural',
 ];
 
 const FUNCTION_NAMES = [
@@ -210,6 +211,17 @@ export default function ({Plugin, types: t}) {
 
                 let name = this.get('name');
 
+                if (name.referencesImport(moduleSourceName, 'FormattedPlural')) {
+                    let {loc} = node;
+                    file.log.warn(
+                        `[React Intl] Line ${loc.start.line}: ` +
+                        'Default messages are not extracted from ' +
+                        '<FormattedPlural>, use <FormattedMessage> instead.'
+                    );
+
+                    return;
+                }
+
                 if (referencesImport(name, moduleSourceName, COMPONENT_NAMES)) {
                     let attributes = this.get('attributes')
                         .filter((attr) => attr.isJSXAttribute());