Allow setting content variables using a special syntax

This is still experimental.

By defining methods starting with `content_for_` and ending in `!`,
variables can be set at render time.

```ruby
class RocketPresenter < Curly::Presenter
  # Same as calling `content_for :rocket_name, "Apollo"` in an
  # ERB template.
  def content_for_rocket_name!
    "Apollo"
  end
end
```