* feat: command to import configuration from a directory
This allows us to keep Superset updated from a repo:
```bash
$ superset import-directory /path/to/configs/
```
For example, I created a simple dashboard with a single chart:
PLACEHOLDER
I then exported it to a file `dashboard_export_20210714T104600.zip` and
unzipped it. After deleting the dashboard, chart, dataset, and database
I imported everything back with:
```bash
$ superset import-directory ~/Downloads/dashboard_export_20210714T104600/
```
I then changed the chart title in `~/Downloads/dashboard_export_20210714T104600/charts/Cnt_per_country_1.yaml` and ran the command again. The chart was succesfully updated:
PLACEHOLDER
* Small fixes
