Add uniq command (#1132)

* start playing with ways to use the uniq command

* WIP

* Got uniq working, but still need to figure out args issue and add tests

* Add some tests for uniq

* fmt

* remove commented out code

* Add documentation and some additional tests showing uniq values and rows. Also removed args TODO

* add changes that didn't get committed

* whoops, I didn't save the docs correctly...

* fmt

* Add a test for uniq with nested json

* Add another test

* Fix unique-ness when json keys are out of order and make the test json more complicated

docs/commands/uniq.rs

@@ -0,0 +1,36 @@
+# uniq
+
+Returns unique rows or values from a dataset.
+
+## Examples
+
+Given a file `test.csv`
+
+```
+first_name,last_name,rusty_at,type
+Andrés,Robalino,10/11/2013,A
+Andrés,Robalino,10/11/2013,A
+Jonathan,Turner,10/12/2013,B
+Yehuda,Katz,10/11/2013,A
+```
+
+```
+> `open test.csv | uniq`
+━━━┯━━━━━━━━━━━━┯━━━━━━━━━━━┯━━━━━━━━━━━━┯━━━━━━
+ # │ first_name │ last_name │ rusty_at   │ type
+───┼────────────┼───────────┼────────────┼──────
+0 │ Andrés     │ Robalino  │ 10/11/2013 │ A
+1 │ Jonathan   │ Turner    │ 10/12/2013 │ B
+2 │ Yehuda     │ Katz      │ 10/11/2013 │ A
+━━━┷━━━━━━━━━━━━┷━━━━━━━━━━━┷━━━━━━━━━━━━┷━━━━━━
+```
+
+```
+> `open test.csv | get type | uniq`
+━━━┯━━━━━━━━━
+# │ <value>
+───┼─────────
+0 │ A
+1 │ B
+━━━┷━━━━━━━━━
+```