Add support to customize nested key delimiter

[devxoul]

Summary

This PR makes it available to use custom nested key delimiter in mapping.

Background

Imagine that we have a JSON dictionary which has nested-dot-containing keys:

{
  "kr.xoul.user": {
    "kr.xoul.name": "Suyeol Jeon",
    "kr.xoul.username": "devxoul"
  }
}

In this case, mapping function would not work properly.

func mapping(map: Map) {
  // this will parse the key as ["kr"]["xoul"]["user"]["kr"]["xoul"]["name"]
  name <- map["kr.xoul.user.kr.xoul.name"]
}

Improvement

Adding delimiter parameter to map subscript.

func mapping(map: Map) {
  // this will parse the key as ["kr.xoul.user"]["kr.xoul.name"]
  name <- map["kr.xoul.user->kr.xoul.name", delimiter: "->"]
}

Notes

• This PR also contains nested key support in ImmutableMappable. (will it better separating to another PR?)

Related Issues

• Resolves Add support to change Separator character  #624