Plugins: Add isEnabled method

Plugins are now disabled at startup time unless there is some relevant
configuration in hydra.conf. This avoids hydra-notify having to do a
lot of redundant work (a lot of plugins did a lot of database queries
*before* deciding they were disabled).

Note: BitBucketStatus users will need to add 'enable_bitbucket_status
= 1' to hydra.conf.

src/lib/Hydra/Plugin.pm

@@ -12,11 +12,15 @@ sub new {
     return $self;
 }
 
+sub isEnabled {
+    return 1;
+}
+
 sub instantiate {
     my ($class, %args) = @_;
     my $plugins = [];
     $args{plugins} = $plugins;
-    push @$plugins, $class->plugins(%args);
+    push @$plugins, grep { $_->isEnabled } $class->plugins(%args);
     return @$plugins;
 }