Allow keeping logs outside of the process cwd

[elbiczel]

In https://github.com/bustardcelly/grunt-forever/blob/master/tasks/forever-task.js you have:

if (this.options().logDir) {
  logDir  = path.join(process.cwd(), this.options().logDir) || logDir;
  logFile = path.join(logDir, this.options().logFile || 'out.log');
  errFile = path.join(logDir, this.options().errFile || 'err.log');
}

I would like to keep my logs outside of the process.cwd(). Especially when the this.options().logDir.startsWith('/'). Could you add support for that please?