Thus, we remove the -y/--yes options, which were kind of unintuitive,
and we add instead `-i/--interactive`. The quickstart commands remain
interactive by default, but can be silenced with `-I/--non-interactive`.
- More concise table of contents
- New intro
- Simpler make commands
- Fix a couple typos here and there
- Get rid of the default github issue template, and start using the
template created online.
Configuration values can be loaded from the system environment by adding
a "TUTOR_" prefix.
Environment values supersede values from the user configuration file, so
that we can set values from the command line with "KEY=VAL tutor config
save --silent" even when KEY is already present in the user
configuration file.
Environment is no longer generated separately for each target, but only
once the configuration is saved.
Note that the environment is automatically updated during
re-configuration, based on a "version" file stored in the environment.
