Hash get value, get default value

---

• Get value of a key
• Get value or nil if the key does not exist
• Get value of a default value if the key does not exist

examples/hashes/get_value.cr

person = {
  "name"   => "Jane",
  "number" => 42,
}
puts person

# Unhandled exception: Missing hash key: "email" (KeyError)
# email = person["email"]
email = person["email"]?
puts email.nil? # true

name = person["name"]?
puts name # Jane

email = person["email"]? || "default@example.com"
puts email.nil? # false
puts email      # default@example.com