Adding Custom Link Formatting

[jordanh]

New glow user here. I wanted to optionally be able to have links formatted as OSC8 links if my terminal supported it.

Generally, there is some demand for custom link formatting (see: #361).

Here is my proposal:

 // Create renderer with custom formatter
 renderer, err := glamour.NewTermRenderer(
     glamour.WithStandardStyle("dark"),
     glamour.WithLinkFormatter(customFormatter),
 )

 // Render markdown
 output, err := renderer.Render(markdown)
 fmt.Print(output)

With formatters being implemented as one of:

// Function-based formatter
customFormatter := ansi.LinkFormatterFunc(func(data ansi.LinkData, ctx ansi.RenderContext) (string, error) {
    // Your custom formatting logic here
    return fmt.Sprintf("CUSTOM: %s -> %s", data.Text, data.URL), nil
})

// Struct-based formatter implementing LinkFormatter interface
type MyFormatter struct {
    prefix string
}

func (f *MyFormatter) FormatLink(data ansi.LinkData, ctx ansi.RenderContext) (string, error) {
    return fmt.Sprintf("%s%s (%s)", f.prefix, data.Text, data.URL), nil
}