Add incubator option for carpentry field. (#542)

Closes #541

Author: tkphd

bin/lesson_check.py

@@ -176,7 +176,7 @@ def check_config(reporter, source_dir):
     reporter.check_field(config_file, 'configuration',
                          config, 'kind', 'lesson')
     reporter.check_field(config_file, 'configuration',
-                         config, 'carpentry', ('swc', 'dc', 'lc', 'cp'))
+                         config, 'carpentry', ('swc', 'dc', 'lc', 'cp', 'incubator'))
     reporter.check_field(config_file, 'configuration', config, 'title')
     reporter.check_field(config_file, 'configuration', config, 'email')