Stop automatically overwriting width and height style information when using the Better Figures and Images plugin.

At the moment, if you have the following figure or image directive

    .. figure:: /images/my-image.png
        :width: 100

and are using the `Better Figures and Images` plugin, then the plugin will open the image, determine its size, and overwrite the `width` attribute you have provided. Clearly, this is not the intended behaviour. Sometimes you may want to manually size images, and in this case there is no need to open the file with PIL at all.

This pull request removes this behaviour by modifying the style information in the `<img>` tag, as opposed to just appending to it.

I have only tested this on my own Pelican installation -- it might be worth trying in at least one other place before merging.