Mutagen-helper

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- Add support for many projects in the same configuration file
- Add auto configuration support to create sessions from subdirectories with no configuration file
- Add `MUTAGEN_HELPER_PATH` environment variable

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- Add missing [expandvars](https://github.com/sayanarijit/expandvars) dependency
- Remove [tinydb](https://github.com/msiemens/tinydb) dependency

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- Add default value support for environment expansion replacements, with
[expandvars](https://github.com/sayanarijit/expandvars)
- Upgrade list command to support short and long listing (`--long` option)

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- Add `project` and `session` arguments to commands
- Fix stuck sessions on `up` command because of invalid stdin/stdout support
- Fix missing project name in output
- Remove dead code

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- Use labels feature introduced in mutagen
[v0.9.0-beta2](https://github.com/havoc-io/mutagen/releases/tag/v0.9.0-beta2). There's no local database anymore
(`~/.mutagen-helper` directory can be removed) ([4](https://github.com/gfi-centre-ouest/mutagen-helper/issues/4))
- Add support for YAML configuration files without dot (`mutagen.yml`
and `mutagen.yaml`) ([3](https://github.com/gfi-centre-ouest/mutagen-helper/issues/3))

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- Enhance error handling of mutagen commands ([5](https://github.com/gfi-centre-ouest/mutagen-helper/issues/5))
- Add support of user input for underlying mutagen
commands ([6](https://github.com/gfi-centre-ouest/mutagen-helper/issues/6))