[proposal] Support _custom

[caibirdme]

There're many cases that user want to use some new types such as JSON in sql. And new types are bound to related function calls. It's hard to enumerate them all, so we should give user a chance to insert some custom part of the sql in the where map.

// PartialSelection
type PartialSelection interface {
  Generate() (string, []interface{})
}

{
  "_custom_xx": typeA_impl_PartialSelection,
  "_custom_asd": typeB_impl_PartialSelection,
}

the key must be prefixed with _custom_ which tells gendry to carefully handle the PartialSelection.

And then, we can add some builtin functions that produce PartialSelection like:

func JSONContains(col, val, path string) PartialSelection

And user can also impl their owns as well

[jiangwei2514]

可以使用 Comparable 接口，与现有的操作符（=,< 等）实现的接口一致
//Comparable requires type implements the Build method
type Comparable interface {
Build() ([]string, []interface{})
}