added new hook before_die (#509)

* added new hook before_die

before die, the script calls this hook in oder to execute extra user's 
operations

* PR-509 -  added new hook before_die

- Added test cases

---------

Co-authored-by: Sveta Smirnova <sveta.smirnova@percona.com>

t/pt-online-schema-change/samples/plugins/before_die.pm

@@ -0,0 +1,20 @@
+package pt_online_schema_change_plugin;
+
+use strict;
+use warnings FATAL => 'all';
+use English qw(-no_match_vars);
+use constant PTDEBUG => $ENV{PTDEBUG} || 0;
+
+sub new {
+   my ($class, %args) = @_;
+   my $self = { %args };
+   return bless $self, $class;
+}
+
+sub before_die {
+   my ($self, %args) = @_;
+   print "PLUGIN before_die\n";
+   print "Exit status: $args{exit_status}\n";
+}
+
+1;