Optionally use no bg colour in place of base00

[arcnmx]

let base16background=none in vimrc to activate

Description

Terminal backgrounds are typically configured to be the same colour as base00, but (most?) terminals can treat them as a separate palette slot where they can be potentially augmented or include transparency.

Checklist

• [ ] I have built the project after my changes following the build instructions using make
  • I only seem to have the node builder in my package manager :(
• [x] I have confirmed that my changes produce no regressions after building
  • it's opt-in so...
• [ ] I have pushed the built files to this pull request

[fnune]

I only seem to have the node builder in my package manager :(

You can install pybase16-builder with pip: pip3 install pybase16-builder