Support SVG images in LaTeX

Hi there, this is Local State.

Motivation

The current Image directive is not enough for svg files. Using img tag loses interactivities, and the width, and height can't be applied for svg files directly. It's better to use style="width:50%" instead.

2 New Options

I'm hoping there could be 2 new options for Image directive.

1. tag
   This would determine the html tag used to render the image. The default value would be img, and we could also use object, iframe and embed as alternatives. Currently, all images are rendered with img tag, which loses some interactivities (e.g., the text in svg file). We could avoid this by using object tag. I see there are some similar work in html4css1 about object_image_types.

   Example:
   ```
   <img src="a.png" width="50%" alt="alt msg">`
   <object data="a.png" style="width:50%">alt msg</object>
   <iframe src="a.png" width="50%" title="alt msg"></iframe>
   <embed src="a.png" width="50%">`
   ```
   

2. embed
   If enabled, this option would embed the image into the html file using base64 encoding.
   While I see there is already the if sentence in the visit_image function.
   if self.settings.embed_images or ('embed' in node):
   The embed_images could be set from command-line or setting, but I can't find how to satisfy the latter condition, since any node doesn't seem to have embed attribute. Could anyone tell me how to enable it? If we can't, then it's necessary to add the embed option, so that we could choose to embed one particular image or not.
   Especially, I notice there exist a todo item for svg embedding. I'd like to implement it if possible. But the first option is still in demand, because most times we don't want our html files to be so large and need separate svg files.

Potential bugs

svg files under <img> tag can't correctly render height and width if svg viewbox information is not contained in the file. It might be better to set the height and width in style indirectly.

Contribution

I would like to fix and implement all these things if possible. But I'm not quite familiar with SourceForge and don't know how to submit a PR like what we do in github.