feat: add branch name

examples/gallery/lib/main.dart

@@ -97,6 +97,7 @@ class _MyHomePageState extends State<MyHomePage> {
             StateLabel(StateLabelStatus.issueOpened),
             StateLabel(StateLabelStatus.issueClosed),
             StateLabel(StateLabelStatus.pullMerged),
+            BranchName('a_new_feature_branch'),
             Text(
               'You have pushed the button this many times:',
             ),

lib/primer.dart

@@ -1,2 +1,3 @@
 export 'src/label.dart';
 export 'src/colors.dart';
+export 'src/branch_name.dart';

lib/src/branch_name.dart

@@ -0,0 +1,23 @@
+import 'package:flutter/widgets.dart';
+import 'colors.dart';
+
+const branchBgColor = Color(0xffeaf5ff);
+
+class BranchName extends StatelessWidget {
+  final String name;
+
+  BranchName(this.name);
+
+  @override
+  Widget build(BuildContext context) {
+    return Container(
+      padding: EdgeInsets.symmetric(vertical: 2, horizontal: 6),
+      decoration: BoxDecoration(
+        color: branchBgColor,
+        borderRadius: BorderRadius.all(Radius.circular(3)),
+      ),
+      child: Text(name,
+          style: TextStyle(color: PrimerColors.blue500, fontSize: 12)),
+    );
+  }
+}