* 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