Set own custom logger module to apply a log output solution for each platform

[yoichiro]

Fixes #337

Abstract

This pull request provides a new feature to set own custom logger module.

Detail

This pull request has some modifications like the following:

• Currently, each log function are defined on the common module. This pull request removes them from the common module. Instead, this pull request introduces a new module named 'logging'.
• The logging module defines three elements: Logger interface, DefaultLogger class and LoggerProxy class.
• The Logger interface defines each log function signature including debug, error, info, warn and deprecate.
• The DefaultLogger class has same features which the current common module is providing.
• The LoggerProxy class is a proxy class. Actually, the logging module exports this instance named logger. The LoggerProxy class delegates each function call to the implementation instance. The DefaultLogger class is one of the implementation, and the initial LoggerProxy instance has the DefaultLogger instance so that this client library can provide same behaviors of the latest version.
• All codes which calls common.<debug, error, warn, info>() are replaced with logging.<debug, error, warn, info>().

Developers can define own custom logger with the Logger interface like the following:

const customLogger = {
  debug(message?: any, ...optionParams?: any[]) {
    // Write a log with arguments by own method/format...
  },
  ...
} as Logger;

And, the developers can specify the own logger above with like the following:

import { dialogflow } from `actions-on-google`;

const app = dialogflow({
  logger: customLogger
});

[yoichiro]

After I received some comments from committers, I thought deeply about whether this feature is really necessary or not. As the result, I reached the recognition that current log outputs are for debug of the SDK itself. That is, I think that I should introduce another mechanism to output logs for analytics and so on rather than I change current log output logic. I close this pull request and the related issue. Thank you for the comments and reviewing!