Add options for userstyle and userscript

[johnfactotum]

Is your feature request related to a problem? Please describe.

Some people want more control over the book's stylesheet. Which is just not possible because when you override stylesheets, things will break. And the UI will eventually be cluttered with hundreds of options.

Describe the solution you'd like

Let the user load a userstyle file, similar to Epiphany. They can tweak the stylesheet to their heart's content at their own risk.