Add backgroundOpacity config option

[fr0stbyt]

Adds a backgroundOpacity config option that allows users to fade the background image toward black by stacking a semi-transparent black overlay using CSS linear-gradient.

• Value range: 0.0 (fully black) to 1.0 (full image, default)
• Backwards compatible — defaults to 1.0 if not set
• Does not affect terminal text or UI elements

Usage:

backgroundOpacity: 0.15

[retrixe]

Would using background-blend-mode be a better approach? Like so:

https://stackoverflow.com/questions/12605908/change-background-image-opacity

Also, I think you've changed the line endings to CRLF, since the diffs are big.