Dictionary Key-Path Subscript

[calebkleveter]

Fixes #565

var dict =  ["key": ["key1": ["key2": "value"]]]
dict[path: ["key", "key1", "key2"]] = "newValue"
dict[path: ["key", "key1", "key2"]] -> "newValue"

Checklist

• I checked the Contributing Guidelines before creating this request.
• New extensions are written in Swift 4.
• New extensions support iOS 8.0+ / tvOS 9.0+ / macOS 10.10+ / watchOS 2.0+.
• I have added tests for new extensions, and they passed.
• All extensions have a clear comments explaining their functionality, all parameters and return type in English.
• All extensions are declared as public.
• I have added a changelog entry describing my changes.