Edit WordPress plugin markup output

[logicelf]

Rendered screenshots overflow the default WordPress theme width. After some experimentation, and having reviewed the latest WordPress Codex docs, I suggest the following changes to the markup that the plugin outputs:

1. Change the wrapping div to a figure element
2. Hard-code that figure element with the wp-block-image class
3. Update the plugin settings page text accordingly

This should allow outputted images to be as compatible as possible with modern WordPress coding standards and the majority of themes, while still allowing users to override the outer and inner elements if necessary.