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)

[antoineco]

@fnune thanks for maintaining this fork! (along with base16-vim which I also switched to) Very useful 👍

@chriskempson do you have anything against adding this? The repo is fairly outdated compared to the "main" base16 theme repo.

[fnune]

My hopes are that if and once @chriskempson is back we can then transfer the repo or the commits.