支持读取额外的hosts文件

方便切换多个固定的hosts文件 :smile:

[example](https://github.com/imyelo/a_a/blob/b351c29bc3281b47d588e3c41f12b86145197c66/example/app.js)
···javascript
var fs = require('fs');
require('o_o')(fs.readFileSync('./hosts').toString()).listen(8890);

// or just
// require('o_o')().listen(8890);
···