add tests file ex.py and test.py...
add 'mutation apply UID'.
do not show mutation during test, but during replay.
v0.3.1 with python 3.7

- Check the database exist.

- play_test_tests: pass extra argument `command` for replay...

- Specify numprocesses only when max workers > 1.

- During `play` store command for re-use in `replay`.

- Count mutation failures.

- replay: re-use command.
mutation: ignore files without coverage.
alpha is the time taken to run the tests
sort files by size before computing mutations...

... it is better, because big files will take a lot of time to
complete, whereas small file will do fast, that way we can maximaze
the time where the pool is busy.

- duplicate source command before using it, to avoid accumulating

- Improve wording.

- Less TODO.

- Less or no need for benchmarks.

- repository is a git repository not the index.
small refactor.

- add missing replay command in docopt.

- fix sampling return values.

- allow float as sampling.

- display success message only in case of success.

- replay: fix max_workers=1 until further notice.

- replay: show mutation once.

- replay: uid is useless.

- replay: lookup the good subspace for failed mutations

- cli: it is possible to pass pytest argument file-or-directory
  without going throught the ceremony of copy pasting the whole

- add tqdm to show progress.

- bump v0.2.9.

- print output to stdout.

- only give a summary of ignored mutations.

- not interesting != interesting.

- drop the delta.isspace() it never happens.

- rename mutation_tess to mutation_pass, reverse again the logic to
  return False when mutation fails.

- PYTEST: no tb, with summary.

- database_open accept a string or Path.

- run returns an integer.

- add replay command to help with debugging.
refactor + black + isort.
replay mockup
store mutation failures.