Add support for markdown in TUI, console, clipboard using pypandoc library

[danschwarz]

As it turns out, the pandoc utility, with pypandoc Python bindings, do a better job converting HTML to markdown than html2text or markdownify libraries. Pandoc is widely used and maintained, in contrast to the other libraries.

In the TUI, statuses are rendered in HTML by urwidgets if available, fallback is markdown (via pandoc if available), and finally plaintext if neither urwidgets nor pandoc are available.

In the console and clipboard, status rendering is in markdown via pandoc, fallback to plaintext if pandoc is not available.

TODO:

1. Command line option or config file option to choose tui status rendering: HTML, markdown, or plaintext.
2. Command line option or config file option to choose console status rendering: markdown or plaintext.
3. Test cases to exercise all three rendering options correctly. At present, test_console.py (correctly) complains because statuses rendered as markdown do not match the expected plaintext rendering.

Here's an example of markdown rendering in the TUI. This is using github-flavored-markdown, FWIW.

This addresses issue #416 .

[danschwarz]

something is wrong with the code in this PR. fixing.