Experiments proof of concept (#4199)

* experiments: add initial experiments.show

* only includes metrics + params (no code/data information)

* experiments: basic `dvc experiments show` functionality

* experiments: clone and run repro inside clone workspace

* repro: add -e/--experiment option

* experiments: hash experiments to identify duplicates

* experiments: update show

* experiments: checkout experiment after running repro

* experiments: add `dvc experiments checkout`

* experiments: add simple `dvc experiments diff` command

* just shows combined output from `metrics diff` and `plots diff` for
  now

* add very simple tests

* add some useful output messages

* experiments: suppress help messages while feature is in development

* experiment related commands will be accessible but hidden from the
  default command help messages

* add simple experiments command tests

* experiments: disable feature by default

* experiments only enabled in test environment (DVC_TEST) or when
  core.experiments config option is true

* use fetch on checkout missing revs

.dvc/.gitignore

@@ -9,3 +9,4 @@
 /pkg
 /repos
 /tmp
+/experiments