Add new_xxx api to parser and formatter

Second try of https://github.com/fluent/fluentd/pull/495.
Note that `new_parser` returns actual XXXParser, not TextParser.
Keep existing instantiation approach for existing plugins.

After merged this PR, I will implement `Fluent::Parser` and `Fluent::Formatter` for plugins.
`Fluent::Parser` provide `parse` and `call` methods. `call` is for backward compatibility.