Remove nobel_prize directory after page build

Nobel prize directory becomes a .git repo during page build, so we need
to delete it before making github pages, otherwise git gits confused.
Small rewrites pending a rethink
NF: add variable substitution from env dict

Get dict from env with variables to substitute, substitue using
jinja-like {{ var }} templating.
NF: add ability of subclasses to add code pre/post

Subclasses can prepend, postpend code to executed code, that does not
show up in the built page.
RF: get / set commit info during runblock code

Use env substitution to set git commit times (and therefore guarantee
stable commit hashes).
Rewriting git tutorial with commit machinery

Lots of unrelated changes in the tutorial though, as I go through.
NF: add section on remotes as backup

Need to expand to cover fetch and pull.
NF: refactor link writing, add short hash links
WIP: more rewriting, explaining remotes
RF: refactor var and link setting into mixins

Make exe_pre and exe_post into options.

Add AddVars directive to get substitution variable from command.
Finished full pass on git tutorial

Maybe it's a little too long.
Some cleanup for less-live tutorial.
RF: refactoring top-level stuff
Cleaning up on reread