Add pretty flag to to md (#2640)

* First draft for adding a `pretty` flag to `to md`

* rustfmt

* Fix Clippy warnings

* rustfmt

* Using Clippy suggestion broken code, reverting and putting in a statement to ignore clippy warning

* Add test for `to md -p`

crates/nu-cli/tests/format_conversions/markdown.rs

@@ -23,3 +23,15 @@ fn out_md_table() {
 
     assert_eq!(actual.out, "|name||-||jason|");
 }
+
+#[test]
+fn out_md_table_pretty() {
+    let actual = nu!(
+        cwd: ".", pipeline(
+        r#"
+            echo '{"name": "joseph"}' | from json | to md -p
+        "#
+    ));
+
+    assert_eq!(actual.out, "|name  ||------||joseph|");
+}