fix: strict unmarshalling

[FUSAKLA]

/fixes #68

Adds a workaround for missing KnownFields option when using Node.Decode as described in yaml feaature request here go-yaml/yaml#460

It is hacky, but unfortunately I see no better way to do this :/

Signed-off-by: Martin Chodur m.chodur@seznam.cz

[FUSAKLA]

After the fix group with field foo: bar returns an arror like

Result: 
  File: examples/rules/rules.yaml - INVALID
    - invalid file examples/rules/rules.yaml: unknown field "foo" when unmarshalling the *unmarshaler.RuleGroup, only supported fields are: name,interval,query_offset,partial_response_strategy,source_tenants,rules,limit