New feature import avro with logical type default values

[enriquecatala]

As discussed here https://github.com/datacontract/datacontract-specification/issues/63

I added the new config support for both avroLogicalType and avroDefault

it supports both import and export from yaml to avsc and vice versa

[simonharrer]

Awesome. Great contribution. Can you add a changelog entry? And we need to document these avro-specific config fields somewhere. Could you add some documentation in the readme?

[enriquecatala]

Awesome. Great contribution. Can you add a changelog entry? And we need to document these avro-specific config fields somewhere. Could you add some documentation in the readme?

done

[simonharrer]

Awesome. Thanks for contributing!