Refactor logger system

[icyleaf]

At first, i just only want add json-formatted logger, for current logger system was not carry it, so make a little refactor same as MIME type system and add JSON-formatted logging.

• [x] refactor logger system same as MIME type system.
  • [x] register logger adapter
  • [x] rename @logger to @writer for disambiguation
  • [x] expose @writer to be a setter
  • [x] JSON-formatted logging support
  • [x] remove Halite::Logger.clear to keep built-in adapters
• [x] use adapter both with name and instance class support!

Before

class CustomLogger < Halite::Logger
  def request(request)
    @logger.info ">> | %s | %s %s" % [request.verb, request.uri, request.body]
  end

  def response(response)
    @logger.info "<< | %s | %s %s" % [response.status_code, response.uri, response.mime_type]
  end
end

Halite.logger(CustomLogger.new)
      .get("http://httpbin.org/get", params: {name: "foobar"})

After

class CustomLogger < Halite::Logger::Adapter
  def request(request)
    @logger.info "| >> | %s | %s %s" % [request.verb, request.uri, request.body]
  end

  def response(response)
    @logger.info "| << | %s | %s %s" % [response.status_code, response.uri, response.content_type]
  end
end

Halite::Logger.register_adapter "custom", CustomLogger.new

Halite.logger(logger: CustomLogger.new)
      .get("http://httpbin.org/get", params: {name: "foobar"})
# Also register name support
Halite.logger(adapter: "custom")
      .get("http://httpbin.org/get", params: {name: "foobar"})