represent the boolean values as per the YAML spec

see http://yaml.org/type/bool.html for implementation detail

src/yaml/from_yaml.cr

@@ -30,7 +30,14 @@ def Nil.new(pull : YAML::PullParser)
 end
 
 def Bool.new(pull : YAML::PullParser)
-  pull.read_scalar == "true"
+  case value
+  when "true", "True", "TRUE", "on", "On", "ON", "y", "Y", "yes", "Yes", "YES"
+    true
+  when "false", "False", "FALSE", "off", "Off", "OFF", "n", "N", "no", "No", "NO"
+    false
+  else
+    raise YAML::ParseException.new("Expected bool, not #{value}", 0, 0)
+  end
 end
 
 {% for type in %w(Int8 Int16 Int32 Int64 UInt8 UInt16 UInt32 UInt64) %}