Bring in history for the lessons/thw-shell/ directory

A sub-part of #79 along the lines of #89.

This pull-request is pre-merged for easy inspection, so don't use the
usual GitHub machinery to merge it.  If folks want this in bc, let me
know, and I'll recreate cf82bdc on top of the then-current bc/gh-pages
branch.

@gvwilson, e3a10f8 (swcarpentry/boot-camps@9ca5e3f) is my best guess
at your starting point for `lessons/shell/tutorial.md`, since that's
what boot-camps had in `shell/Readme.md` when you created 2cc715f.
The Markdown content is not identical though, so let me know if there
are some intervening commits that I'm missing.