feat: add WithLongRenderer option

[bdsoha]

Fang renders a command's Long/Short as plain styled text, and there's no hook to transform it first. So if you want Markdown in your long descriptions, there's nowhere to render it — the raw **bold** and - bullets just print as-is.

This adds a small WithLongRenderer(func(string) string) option. The renderer receives the raw long text and returns what should be displayed; a nil renderer (the default) leaves everything exactly as it is, so it's non-breaking. It's threaded through helpFn → writeLongShort and applied right before the existing styles.Text styling.

Typical use is rendering Markdown through glamour:

r, _ := glamour.NewTermRenderer(glamour.WithAutoStyle())

fang.Execute(ctx, root, fang.WithLongRenderer(func(s string) string {
    out, err := r.Render(s)
    if err != nil {
        return s
    }
    return out
}))

I kept it as a generic callback rather than a glamour-specific option, to avoid pulling glamour into fang's dependency tree — but if you'd rather bake it in (say a WithMarkdown() that wires glamour directly), I'm happy to switch to that instead.

Added a test covering that the renderer receives the raw text and its output lands in the help screen.

[bdsoha]

@charmcrush Hey, had a chance to look at this?