feat: expose fileList to beforeUpload

[benjycui]

Close: ant-design/ant-design#4015

如下可以限制最多上传三个文件：

function beforeUpload(file, fileList) {
  const shouldBeUpload = this.state.fileList.concat(fileList)
    .slice(0, 3).indexOf(file) > -1;
  // TODO
}

[coveralls]

Coverage decreased (-1.05%) to 30.0% when pulling a1fc952 on feat-beforeUpload into 36b7d1a on master.

[benjycui]

ping~

[shepherdwind]

npm owner 加你了，自己发一个版本吧

[benjycui]

2.3.0