feat(ajax): auto read mock data on ajax request

`GET /user/profile.json` on `/home?__scene=default` page, should try to read `/user/profile.json/default.js` mock data file.
- exists, return the mock data
- not exists, return normal data

Closes #5
Merge pull request #7 from koajs/ajax-mock

feat(ajax): auto read mock data on ajax request