Video embed support

[emad-elsaid]

I noticed the library doesn't support video embed. I overcome this limitation by using the following formatter

import (
	"fmt"
	"io"
	"strconv"

	quill_html "github.com/dchenk/go-render-quill"
)

func customFormats(keyword string, op *quill_html.Op) quill_html.Formatter {
	if keyword == "video" {
		return &VideoFormatter{link: op.Data}
	}

	return nil
}

type VideoFormatter struct {
	link string
}

func (v *VideoFormatter) Fmt() *quill_html.Format {
	return nil
}

func (v *VideoFormatter) HasFormat(op *quill_html.Op) bool {
	return op.Type == "video"
}

func (v *VideoFormatter) Write(buf io.Writer) {
	fmt.Fprintf(buf, `<iframe class="video" src=%s></iframe>`, strconv.Quote(v.link))
}

I use html, err := quill_html.RenderExtended([]byte(content), customFormats)to render the video link inside an iframe. worked with youtube.

I couldn't edit the wiki so I thought it'll be useful to document it in the issues here.