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.
Replace all make commands by a single "tutor" binary. Environment and
data are all moved to ~/.tutor/local/share/tutor. We take the
opportunity to add a web UI and revamp the documentation.
This is a complete rewrite.
Close #121.
Close #147.
