Images in high dpi show with the wrong size

### Title
Images in high dpi show with the wrong size

### Functional impact
I am using high dpi images in my docs so they look clear for people reading them from high dpi displays. But I can 't find a way to make the image size correct, or to at least manually set the image size without resorting to using an html img tag which I would prefer not to use.

### Minimal repro steps
Try writing in a document:
```
```
and supply the following image:

### Expected result
I would expect to see the image at the size corrected by its dpi, that is half of its size (which is how it looks on the screen).

### Actual result
The image has double the size.

### Further technical details
I think it would be very nice to have DocFX automatically write the **width** and **height** attributes in the tags it generates from markdown, compensating by the dpi of the image. Right now it seems like it doesn't write the height and width at all, which not only is bad because for high dpi images, but in general because if the image fails to load, the browser won't be able to know the expected dimensions of the image. Image includes alt-text, should also include the dimensions for the same reasons.

If not possible to automatically compensate for image resolution, it would be at least nice to be able to define the height and width of the images manually in markdown, without having to use html. But of course the best would be for DocFX to take care of the full process: I took a screenshot on a high dpi display and inserted it in the markdown document. I would expect it to look right without any more involvement.

Also supporting high dpi images is a must in a world where all mobile devices are high dpi, and increasingly PCs too.