Implement feature derivation

[typhoonzero]

Implement feature derivation following this doc: https://github.com/sql-machine-learning/sqlflow/blob/develop/doc/feature_derivation.md.

Should depend on IR work at: #805

• add feature_derivation.go to do the job, output a *codegen.FeatureColumn for all columns in the SELECT statement.
• call feature_derivation.go in ir_generator.go to generate derevated feature columns.
• Use feature_derivator.go in all codegen:
  • tensorflow
  • ALPS
  • xgboost
  • ElasticDL

[tonyyang-svail]

@typhoonzero I believe the current checklist is outdated since all codegens will be depend on IR.

[typhoonzero]

Done updated.

[tonyyang-svail]

Done and closing.