Port skip, skip while and skip until commands (#380)

* Add `Skip` command

* Add `SkipUntil` sub-command

* Add `SkipWhile` sub-command

* Add and use `Expression::as_row_condition_block`

crates/nu-command/src/default_context.rs

@@ -116,6 +116,9 @@ pub fn create_default_context() -> EngineState {
             Select,
             Shuffle,
             Size,
+            Skip,
+            SkipUntil,
+            SkipWhile,
             Sleep,
             Source,
             Split,