[proposal] Support _custom  #150

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