Automate weekly repo refresh with GitHub Actions, change & document build process to use pybase16-builder

[fnune]

I've forked the repository to my own: https://github.com/fnune/base16-shell

I've also submitted the changes here as PR to see if they get attention. If they're merged, I'll archive my fork and link back here!

This PR:

• Ensures the repository remains up to date on a weekly basis, automatically, using GitHub Actions
• Uses pybase16-builder because it's easier to figure out how to use it
• Documents how to build the repository on your own and keep it up to date (not necessary as a user, you just need to git pull)