Add extendedDiscussion to CommandConfiguration

[realrajaryan]

Resolves #159.

Adds an extendedDiscussion property to CommandConfiguration that renders after the argument, option, and subcommand sections in help output.

This gives command authors a place to put supplementary information i.e. usage examples, notes etc.

Checklist

• I've added at least one test that validates that my change is working, if appropriate
• I've followed the code style of the rest of the project
• I've read the Contribution Guidelines
• I've updated the documentation if necessary

[rauhul]

Can you add an extended discussion to one of the example tools so we can see how the generated content changes?

[realrajaryan]

Can you add an extended discussion to one of the example tools so we can see how the generated content changes?

added it to the math example

[rauhul]

does it make sense for this to be below the options for the markdown generator? I think before or after could make sense, wdyt?

[realrajaryan]

oh ye, I'll move it after the subcommands to match the CLI help layout