Add ability to disable specific elements in a ComboBox

[dylan-poole]

What changed:

• feat: Allow users to add the attribute disabled to Listbox::Options.
• feat: Added CSS for disabled options in the listbox.
• feat: add options collection for list boxes that displays disabled options but excludes them from selection.
• test: Added a disabled options combobox_tag to the disabled.html.erb view.

Additional Information

• We decided it was best to only support disabled options when passing a hash to combobox_tag.
• This can be further expanded if we would like to look into creating a helper function for combobox_options, like options_for_select.
• This PR resolves discussion 126.

Testing Instructions

1. Run the dummy app and navigate to: /disabled.
2. Click on the ComboBox labeled, State (with disabled options).
3. Expect all states that start with the letter 'A' to be disabled.
4. Ensure you cannot select them and that you cannot navigate to them.
5. Ensure you cannot get to any disabled elements by using the Home or End key press.
6. Ensure you cannot get to any disabled elements by using the search feature.

[dylan-poole]

@josefarias As promised, sorry for the delay I kept running into to other ways you could select a disabled element! Let me know if there are any areas that need to be tweaked, it was great meeting with you at RailsConf!