Add template functions for better handling the backend with rendering

[EkoJR]

trafficstars

Similar to how WooCommerce's template concept is implemented.

• There's 2 main template functions; get_template & get_template_part.
  • The get_template: initial or isolated block of render code.
  • The get_template_part: blocks of code often repeated within the same template; can also pass variables.
• Can be extended to the current theme, or extended plugin.