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/GithubStatus.pm

@@ -8,6 +8,11 @@ use LWP::UserAgent;
 use Hydra::Helper::CatalystUtils;
 use List::Util qw(max);
 
+sub isEnabled {
+    my ($self) = @_;
+    return defined $self->{config}->{githubstatus};
+}
+
 sub toGithubState {
     my ($buildStatus) = @_;
     if ($buildStatus == 0) {