Create shortcode for bash commands

[smythp]

This was raised in PR #218.

When giving examples of bash (or other REPL) commands, often you want to include the $ or prompt for context to distinguish from output, but then the $ or prompt must be manually removed before pasting.

It might make sense to create a Hugo shortcode to handle this. It could use pseudo-elements to show the $, which would make them visible but not (by default on most browsers) copyable. On screen readers the content would appear as it does now (most screen readers treat pseudo-elements as elements), which I think is acceptable.

I think the potential downsides here are a lack of simplicity, and also making future migrations more difficult, as we'd be using more Hugo-specific features. However, I think this would improve the user experience. I'd be happy to write the shortcode if we think it's a good idea.