Define a CLI style guide

[zanieb]

We should determine and document our preferred styling for CLI output so we can be consistent.

Content may include:

• Use of backticks (`)
• Use of colons (:)
• Use bolding and dimming
• Capitalization
• Preferred terminology
• Meaning of colors
• When progress bars are needed
• When lines should be ephemeral
• If messages should be tracing logs or written directly
• When stderr or stdout should be used
• Error message styling
• When capitalization should be used in error/log messages