SVG: fall-back images

[ddediu]

• make them optional
• try to embed them in the HTML document
• try to generate them using base R and not rsvg (see #159)

[ddediu]

• they are optional
• they can be JPEG, PNG or WEBP
• they can be embedded (as base64) in the HTML (JPEG and PNG only)

[ddediu]

Fixed (except for using baseR for placeholder image #159 )