To html and to md (#453)

* MathEval Variance and Stddev

* Fix tests and linting

* Typo

* Deal with streams when they are not tables

* First draft of these commands

* To MD

* To md and to html

* Fixed cargo and to_md

* `into_abbreviated_string` instead of `into_string`

* Changed how inner tables are displayed

crates/nu-command/src/default_context.rs

@@ -164,6 +164,8 @@ pub fn create_default_context() -> EngineState {
             ToToml,
             ToTsv,
             ToCsv,
+            ToHtml,
+            ToMd,
             Touch,
             Uniq,
             Use,