Dataframe feature (#361)

* custom value trait

* functions for custom value trait

* custom trait behind flag

* open dataframe command

* command to-df for basic types

* follow path for dataframe

* dataframe operations

* dataframe not default feature

* custom as default feature

* corrected examples in command

crates/nu-command/src/viewers/table.rs

@@ -107,6 +107,11 @@ impl Command for Table {
                 .into_pipeline_data())
             }
             PipelineData::Value(Value::Error { error }) => Err(error),
+            #[cfg(feature = "custom")]
+            PipelineData::Value(Value::CustomValue { val, span }) => {
+                let base_pipeline = val.to_base_value(span)?.into_pipeline_data();
+                self.run(engine_state, stack, call, base_pipeline)
+            }
             x => Ok(x),
         }
     }