Allow concise configuration of logger backends.

This allows configuration of a parsoid logger in localsettings.js like:

  parsoidConfig.loggerBackend = {
    name: ':Logger.bunyan/BunyanLogger',
    options: {
      name: 'Parsoid tests',
      level: 'trace',
      stream: process.stderr
    }
  };

The leading colon in the name signified a logger defined in Parsoid itself;
3rd-party loggers could be invoked by using the package name before the
slash.

Change-Id: I398985260f3b1bd69c33f7cc89701f1586b2195a