Add support to evaluate VW models using Skater

[pramitchoudhary]

This issue is to enable support for interpreting models built using VW. It might be interesting to compare VW models with other models in terms of performance - sklearn, h20

[aikramer2]

@pramitchoudhary for this, i dont see what the limitation would be, youd just need to provide input_formatter/output_formatter and a callable. Would you like to an add an example?

[pramitchoudhary]

@aikramer2 ya ideally there should not be an issue. The output from vw is a little different compared to other frameworks, so just need to check on that. Also, was hoping to revisit our philosophy on providing some intuitive wrappers for input_formatter/output_formatter with this one. May be we can provide some custom input/output formatter for well supported and consistent libraries. What do you think ?