Make output folder configurable

## The issue

The deployment folder is always `assets/img/shields`. However, not all blogs employ the same principle - it could store images in the `assets/image` directory.

## Potential fix

Expose a config value (in `_config.yml`) to control this behavior, for example:

```yaml
shields_io:
output: "assets/img/shields"
```

Start with _config.yml and locate the plugin code that currently uses assets/img/shields as the deployment folder. Trace how configuration is read and how the output path is selected. Done means a shields_io.output setting can select another folder, while the existing default continues to work.

Written by the indexing model from the issue text.