Allow logger to add transports (#2363)

- Move all of the winston logic from FileLoggerAdapter to WinstonLoggerAdapter
- Export WinstonLoggerAdapter so it can be sublcassed
- Expost the ability to add adittional transports to logger
- Import FirehoseLoggerAdapter alongside other adapters so it can be configured.

spec/Logger.spec.js

@@ -0,0 +1,18 @@
+var logger = require('../src/logger');
+var winston = require('winston');
+
+class TestTransport extends winston.Transport {
+  log(level, msg, meta, callback) {
+    callback(null, true);
+  }
+}
+
+describe('Logger', () => {
+  it('should add transport', () => {
+    const testTransport = new (TestTransport)({});
+    spyOn(testTransport, 'log');
+    logger.addTransport(testTransport);
+    logger.logger.info('hi');
+    expect(testTransport.log).toHaveBeenCalled();
+  });
+});