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.
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.
