refactor: add `KnexBuilder`

为了实现 `knex` 的链式调用。
现在必须 `model().find()...` 这样进行调用
add: `through` support
fix: ignore null value in `find` method
chore: update to `0.1.0`
refactor: method `relationBuilder` => `buildRelation`

And add some comment
refactor: not necessary to get knex obj by calling Models

使用方法变回 `0.0.6` 的方式，不必写成 `Model().find()`
chore: update `0.1.1`