Allow custom attributes to be created and maintained from a file

Small feature to support the recent custom attributes feature added to Rusic.

- [x] Create a themes custom attributes from a Yaml file
- [x] Update existing attributes (depends on https://github.com/rusic/rusic/pull/428)

Example `attributes.yml` file:

```yaml
my_custom_attribute_key:
  value: "Custom attributes rule!!"
  help_text: "Declare how much you admire custom attributes"
```