Added autoloading of secrets when accessing value

Off by default, this change adds an `autoload` method to the `Genesis::Secret`
class that will load the secret if it hasn't been loaded yet. This is useful
for secrets that are not needed in all cases, but are needed in some cases.

The Secret::Plan class has been updated to add an autoload method that will
turn on autoloading for all secrets in the plan as a convenience.

lib/Genesis/Env/Secrets/Plan.pm

@@ -206,6 +206,12 @@ sub filter {
 }
 
 # }}}
+# autoload - load secrets on demand for their values {{{
+sub autoload {
+	my $self = shift;
+	$_->{autoload} = 1 for ($self->secrets);
+	return $self;
+}
 # validate - validate the secret definitions, and any dependencies {{{
 sub validate {
 	my $self = shift;

lib/Genesis/Secret.pm

@@ -28,6 +28,7 @@ sub new {
 			path => $alt_path || $path,
 			definition => $args,
 			loaded => 0,
+			autoload => 0,
 			%src
 		}, $class);
 	}
@@ -66,6 +67,11 @@ sub load {
 	$self->plan->store->read($self);
 }
 
+sub autoload {
+	my ($self) = @_;
+	$self->load unless !$self->{autoload} || $self->loaded;
+}
+
 sub exists {
 	my ($self) = @_;
 	return 1 if $self->loaded && $self->has_value;
@@ -113,8 +119,8 @@ sub all_paths     {($_[0]->path)}
 sub full_path     {$_[0]->base_path.$_[0]->path}
 sub default_key   {undef}
 sub definition    {$_[0]->{definition}}
-sub value         {$_[0]->{value}||$_[0]->{stored_value}}
-sub has_value     {defined($_[0]->{stored_value})}
+sub value         {$_[0]->autoload; $_[0]->{value}||$_[0]->{stored_value}}
+sub has_value     {$_[0]->autoload; defined($_[0]->{stored_value})}
 sub loaded        {$_[0]->{loaded}}
 sub missing       {!$_[0]->has_value}
 sub set_plan      {$_[0]->{plan} = $_[1]; $_[0]}