Add ColumnWiseEncoder

[yiweny]

So we can assign a different encoder for different columns.

[weihua916]

Thanks for raising this! Let me take on this. I think a promising approach is to add an argument, col_encoder_dict, in StypeWiseEncoder so that it uses the given StypeEncoder for specific columns while using the default StypeEncoder (specified in stype_encoder_dict) for the rest.

We can support ColumnWiseEncoder, but it sounds a bit overkill to specify the encoder for each column name.