add encoding for configuration
write document for config.encoding
consider a case of "site" is unavailable in Convertible
read_yaml to accept optional parameter to override defaults
Make TestConvertible to pass with utf-8 encoding
Pass encoding parameter as string, not an object
Invoke File.read with or without options depends on Ruby version

- Extract option fetch method as a separate method
- Added File.read_with_options method to use
- With performance fix
Documentation update for the encoding configuration
Merge pull request #1449 from shigeya/config-encoding-and-yaml-opts

Adding "encoding" configuration (4th version)