Foliantcontrib-utils-combined-options

- Fix: default dict was overriden after the first use.
- Allow to supply the list of priorities instead of just one priority.
- Priority for those option dictionaries, which are not mentioned in the `priority` param, are now defined by the order dictionaries are defined.

- defaults now actually supply default value (before they were only used for validation)
- add None to possible type validation in `val_type`.

- Fix `validate_exists` validator.

- Add `validate_exists` validator.
- Add `rel_path_convertor`.

- Add check for required params.
- Add val_type validator to check for param types.
- Allow to set values in Options objects

- support PyYAML 5.1