Allow adding bits to the project descriptor from the command line

Having included `ruamel.yaml`, now the round trip parsing and edition of yaml is possible so this task should be doable with something like:

```
cpm install --save :
```

Start at the CLI handling for `cpm install` and the project descriptor parsing that uses `ruamel.yaml`. Trace how dependencies are currently installed and how the descriptor is loaded and written. Done means `cpm install --save :` adds the requested bit and preserves the descriptor's YAML content.

Written by the indexing model from the issue text.