Use cli themes

[davidchall]

Currently, printing a template uses hardcoded colors. But the appropriateness of these colors depends on the user's terminal theme. For example, if they use dark mode and we hardcode black text, then they can't see the text.

The solution is to use custom cli classes. Then the user can specify how they want each cli class to be rendered using options(cli.user_theme). See example. The custom cli classes should be:

• jinjar_text
• jinjar_variable
• jinjar_block
• jinjar_comment

[davidchall]

This is currently blocked by r-lib/cli#491.