Loader

Add possibility of loading file for rules

```
name: 'example'
mapping: 'mapping'
header_size: 2
structure_rules: { max_columns: 20 }
rules:
  - { position: [1,1], key: 'first_name' }
  - { position: [2,1], key: 'first_name' }
```
Prefer double-quoted strings unless you need single quotes to avoid extra backslashes for escaping.