Support short-hand partial rendering syntax

This PR updates a few things, but mainly adds support for short-hand partial rendering.

Before, only the following syntax was supported:

```ruby
<%= render partial: 'item', collection: @items, cache: true %>
```

Now, this is also supported:

```ruby
<%= render @items, cache: true %>
```

Few other minor changes:

* Changed `is_a?(Proc)` to `respond_to?(:call)` for a bit more flexibility.
* Enabled using `cache: false` for easy collection caching toggling.
* Removed some trailing whitespace.
* Added ruby syntax highlighting to docs.

Let me know if you want any of these extra commits removed.