Ported the between_objects_gcode custom G-code blocks,

thanks @lordofhyphens, https://github.com/alexrj/Slic3r/pull/3275

Improved handling of custom G-code blocks: Slic3r will try to extract
the target extruder and bed temperatures from the custom G-code blocks.

t/custom_gcode.t

@@ -1,4 +1,4 @@
-use Test::More tests => 40;
+use Test::More tests => 41;
 use strict;
 use warnings;
 
@@ -210,4 +210,13 @@ use Slic3r::Test;
     }
 }
 
+{
+    my $config = Slic3r::Config::new_from_defaults;
+    $config->set('complete_objects', 1);
+    $config->set('between_objects_gcode', '_MY_CUSTOM_GCODE_');
+    my $print = Slic3r::Test::init_print('20mm_cube', config => $config, duplicate => 3);
+    my $gcode = Slic3r::Test::gcode($print);
+    is scalar(() = $gcode =~ /^_MY_CUSTOM_GCODE_/gm), 2, 'between_objects_gcode is applied correctly';
+}
+
 __END__