In text, display model list in markdown table format

[JasonWeill]

Problem

As @dlqqq noted in #100, in the results of %ai list, it may be possible to use libraries like py-markdown-table to generate markdown tables declaratively, based on a data structure, instead of imperatively, as we currently do. Some of our logic is declared twice, once for text and once for markdown format.

Proposed Solution

Use py-markdown-table, or a similar library, to generate markdown for the model list.

Use markdown-style tables for text output.

Remove remaining helper functions that use different logic for text and markdown parts.