Add buildQueued plugin hook

src/lib/Hydra/Plugin.pm

@@ -20,6 +20,11 @@ sub instantiate {
     return @$plugins;
 }
 
+# Called when build $build has been queued.
+sub buildQueued {
+    my ($self, $build) = @_;
+}
+
 # Called when build $build has started.
 sub buildStarted {
     my ($self, $build) = @_;

src/lib/Hydra/Plugin/GithubStatus.pm

@@ -69,6 +69,10 @@ sub common {
     }
 }
 
+sub buildQueued {
+    common(@_, [], 0);
+}
+
 sub buildStarted {
     common(@_, [], 0);
 }