Add buildStarted plugin hook

src/lib/Hydra/Helper/PluginHooks.pm

@@ -5,8 +5,21 @@ use Exporter;
 
 our @ISA = qw(Exporter);
 our @EXPORT = qw(
+    notifyBuildStarted
     notifyBuildFinished);
 
+sub notifyBuildStarted {
+    my ($plugins, $build) = @_;
+    foreach my $plugin (@{$plugins}) {
+        eval {
+            $plugin->buildStarted($build);
+        };
+        if ($@) {
+            print STDERR "$plugin->buildStarted: $@\n":
+        }
+    }
+}
+
 sub notifyBuildFinished {
     my ($plugins, $build, $dependents) = @_;
     foreach my $plugin (@{$plugins}) {

src/lib/Hydra/Plugin.pm

@@ -20,6 +20,11 @@ sub instantiate {
     return @$plugins;
 }
 
+# Called when build $build has started.
+sub buildStarted {
+    my ($self, $build) = @_;
+}
+
 # Called when build $build has finished.  If the build failed, then
 # $dependents is an array ref to a list of builds that have also
 # failed as a result (i.e. because they depend on $build or a failed