Implement an `Array` argument type

[GuySartorelli]

Right now if you want to pass an array into a method or control block from inside a template, you can't do that.

Examples where this might be useful include:

• Deferring a script that you're adding through the template

  <% require javascript('.../my-file.js', ['defer' => true] %>
  

• Passing an array to some method

  $MyMethod(['one', 'two', 'three'])
  

Currently these would probably be parsed as strings, which means #20 should probably be implemented first - though this could be implemented independently.

Either way this would be a breaking change without a feature flag in front of it.