Doit-api

* Multiline string command actions are now interpreted as to be concatenated into the same shell command using `&` (windows) or `;` (linux). This allows several commands to leverage each other, for example `conda activate` + some python execution. Fixes [6](https://github.com/smarie/python-doit-api/issues/6)

* New `cmdtask` similar to `pytask` but to create shell commands. Fixes [5](https://github.com/smarie/python-doit-api/issues/5)

* New `doit_config()` utility method to generate a valid `DOIT_CONFIG` in its caller module. Fixes [1](https://github.com/smarie/python-doit-api/issues/1)

* The `task` API was not compliant with the `num_process` option in the configuration, enabling multiprocess execution of doit. In order to fix the issue (an issue with pickle), `task` was split into two: `pytask` is the decorator, and `task` is the creator. Fixed [3](https://github.com/smarie/python-doit-api/issues/3)

* Initial release with `task`, `taskgen`, `title_with_actions` and `why_am_i_running`.