fix #50: add skipHook to skip fields of object when serializing

[pietroppeter]

fix #50 by allowing to declare a skipHook to skip fields. works at compile time.
also added a new test file with a test case and a new hook section in the readme:

proc skipHook*() Can be used to skip fields when serializing an object

If you want to skip some fields when serializing an object you can declare a skipHook*()

type
  Conn = object
    id: int
  Foo = object
    a: int
    password: string
    b: float
    conn: Conn

proc skipHook*(T: typedesc[Foo], key: static string): bool =
  key in ["password", "conn"]

var v = Foo(a:1, password: "12345", b:0.5, conn: Conn(id: 1))
let s = v.toJson()

Gives us:

"{"a":1,"b":0.5}"

[pietroppeter]

(alternative to #51)

[treeform]

I think your PR fails to due to issue: nim-lang/Nim#21317

[ringabout]

fixed => nim-lang/Nim#21320

[pietroppeter]

If CI is approved I would expect it now to be green. I also fixed the conflict with master.

[pietroppeter]

CI is green and no conflicts with master. Any thoughts?