Add Lucky::CookieJar#[] and #[]=

[grepsedawk]

Adds easier access to cookies by adding the typical key/value syntax
for cookies get/set operations

In the future, this could potentially even implement all of Enumerable({K, V}), but
these methods are super helpful as is to provide hash-like access to cookies

[matthewmcgarvey]

The types need to be specified. Otherwise looks good

[grepsedawk]

The types need to be specified

Ah cool, I thought being a direct alias the implicit types would be fine.
Will change that.