[custom] support command to return size

Sometimes the thing to measure is not available as file or folder (or that path isn't hardcoded), and one might want to define a shell command to retrieve it. An example are docker image sizes where you have to run

```
docker image inspect --format='{{.Size}}'
```

But that might be better covered by #45.

Might be a new config `sizeScript`.

Compare this request with issue #45 first, since the body says it may cover the same case. Then inspect how sizewatcher handles configuration and measures paths, and define the expected behavior for a sizeScript such as the Docker image inspection command. Done means the command-based size source has clear configuration and verified behavior.

Written by the indexing model from the issue text.