Add support for state file

[Actimel]

Comes up from keboola/db-extractor-mysql#81 (comment)

Would be nice to have getter for state file. Besides there could be also method for writing into outcomes state.

[tomasfejfar]

Už tam byla issue #46 ;)

[Actimel]

👍 Nechapu jak jsem to mohl prehlednout, kdyz ty issue jsou jen 3.