Add support for Markdown in TUI, console, clipboard output

[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:

• Command line option or config file option to choose tui status rendering: HTML, markdown, or plaintext.
• Command line option or config file option to choose console status rendering: markdown or plaintext.
• Test cases to exercise all rendering options correctly. At present, we are testing only the plaintext rendering in console.

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

This addresses issue #416 .

[danschwarz]

Switched it to default to plaintext (as it was originally) and use the -md / --markdown option to render as markdown. Makes more sense this way.