Make CSS/scripts in `html_render_diff` customizable

[Mr0grog]

Some researchers have used this package in their work, but need to be better able to customize the CSS and scripts that are automatically inserted into the diff. Right now, you can customize the colors via the DIFFER_COLOR_INSERTION and DIFFER_COLOR_DELETION, but that’s it.

There are probably a lot of ways to do this and a few open questions to answer:

• Are we adding to the existing styles/scripts or replacing them?
• Should this also be available via env vars? Presumably they would specify the path to a style or script file to include.
• Should this be available through the server? Obviously (I think?) it would be available as an argument to the diff function (probably pass in a style/script string, rather than path to file).

/cc @lesleyodu for any additional feedback/input here.