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/default_context.rs

@@ -143,6 +143,9 @@ pub fn create_default_context() -> EngineState {
         #[cfg(feature = "plugin")]
         bind_command!(Register);
 
+        #[cfg(feature = "dataframe")]
+        bind_command!(OpenDataFrame, ToDataFrame);
+
         // This is a WIP proof of concept
         bind_command!(ListGitBranches, Git, GitCheckout, Source);