Feature: Add support for images.

Every entry can have an image associated with it. The width field specifies the width of the image as displayed, as a percentage: 100 will display the image with a Bootstrap img-fluid class, i.e., it will potentially take up the whole width. Values between 0 and 100 display the image flush right with an img-thumbnail class (with a little border). Negative values up to -99 display the image flush left in a similar way.