[BUG] image api 与 mockjs 不兼容

[wll8]

PS: 我叕来提 issue 了~

问题简述
根据 better-mock 的 readme.md 上描述：

100% 兼容 Mock.js。

但是根据 mockjs 的使用文档以及运行结果看来, 与 better-mock 是不相同的.

参考:

• better-mock image: http://lavyun.gitee.io/better-mock/document/random/image.html#random-image
• mockjs image: http://wll8.gitee.io/mockjs-examples/#Image

复现步骤

Mock.mock(`@image(200x100, #ff0000)`)
// => "https://iph.href.lu/200x100?bg=&fg=&text=#ff0000"

Mock.mock(`@image(200x100, #ff0000)`)
// => "http://dummyimage.com/200x100/ff0000"

期望结果
api 与 mockjs 保持一致.

Mock.mock(`@image(200x100, #ff0000)`)
// => "https://iph.href.lu/200x100?bg=ff0000"
// 或
// => "http://dummyimage.com/200x100/ff0000"