Read a value by path

Implement get(obj, path, defaultValue), in the style of lodash's get. It reads a value deep inside obj and returns it, or returns defaultValue when the value isn't there.

The path is either:

• a string such as "user.posts[1].title". Dots separate keys. [1] is another way to write the key 1, so "user.tags.0" also works. Keys never contain dots or brackets themselves.
• an array of keys such as ["user", "posts", 1, "title"].

Rules:

• If any step reaches null or undefined before the path ends, return defaultValue. Never throw.
• If the final value is undefined, return defaultValue. Any other value is returned, even 0, "", false or null.
• Properties of primitives count: "user.name.length" on "Ada" gives 3.
• An empty path ("" or []) returns defaultValue.
• When defaultValue is left out, it is undefined.