Closes #161

While I believe that a Model has no business knowing anything about views, @andrewerlanger's desire to eliminate unnecessary HTTP requests is highly reasonable. Therefore, instead of attempting to further complicate the enable_updates method with the potential for multiple calls that each target different views, this PR gives the updates_for helper the ability to optionally filter out messages which don't contain the right keys.

<%= updates_for User.first, only: "name" do %>
  <p><%= rand(1..1000) %></p>
<% end %>

User.first.update name: "Bob" # triggers refresh
User.first.update age: 43, name: "Steve" # triggers refresh
User.first.update age: 43 # does not trigger refresh

Any broadcasts which update individual model instances now transmit an array of attribute names that were updated:

ActionCable.server.broadcast(identifier, {changed: model.previous_changes.keys})

The only option currently accepts only a simple string that exactly matches the name of an attribute. There's no checking to ensure that it is a valid attribute, and there's no support for and/or-ing multiple attributes. I don't think that we need this feature to be Turing complete to be Just The Right Amount of functionality.

Note that this feature has no impact on Model messages (create/destroy) because it only works on after_update callbacks for individual model instances.