Handle MOJO_CONFIG
Allow to use an other configuration file specified with the env variable MOJO_CONFIG. See https://framagit.org/luc/lstu/commit/ed47a7dcbd5e81d2913e76b876721062695e2c91 and https://framagit.org/luc/lstu/commit/03ff8837e8e386ca9c4a95c6948ec71d19146958