Add pluck method

[andrykonchin]

Implemented a new method Model.pluck which works similar to ActiveRecord's pluck method.

It's like the .project method but returns array of attributes instead of models so should be cheaper and faster. Another difference is that .project returns a chain but .pluck a final collection.

Examples:

User.pluck(:first_name) # => ['Alice', 'Bob', ...]
User.pluck(:first_name, :last_name) # => [['Alice', 'Cooper'], ['Bob', 'Marley'], ...]