html, json, text, xml controller helper methods

[drujensen]

The html, json, text and xml helpers simplify setting the response headers.

Tasks:

• Create a new module in the controllers folder that ports over the following helpers from Kemalyst: https://github.com/kemalyst/kemalyst/blob/master/src/kemalyst/controller.cr#L49-L92
• Change the macros to def's.
• Include the module in the controller base class.

Acceptance Criteria:

• The ability to use templates to render text, xml, html, json
• The proper content-type and response code is set