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/RunCommand.pm

@@ -5,6 +5,11 @@ use parent 'Hydra::Plugin';
 use experimental 'smartmatch';
 use JSON;
 
+sub isEnabled {
+    my ($self) = @_;
+    return defined $self->{config}->{runcommand};
+}
+
 sub configSectionMatches {
     my ($name, $project, $jobset, $job) = @_;