Template support

[cam-stitt]

It would be great for rise to have the ability to load another template via configuration files. They could allow for advanced configuration, including file composition and repeatable templates. Current thinking is to:

• Migrate var argument to --config, -c to load a general configuration file that includes both variables and templates
• Support both inline templates as well as loading from a file path
• Interpolation function such as render("template_name", ctx) to allow for dynamic rendering
• The second argument would be an optional map to provide additional context. This would allow passing in data from another variable at run-time