Delete study subcommand

[c-bata]

refs optuna/optuna#524

$ ./bin/goptuna create-study --storage sqlite:///example.db --study study
study
$ sqlite3 example.db 
SQLite version 3.24.0 2018-06-04 14:10:15
Enter ".help" for usage hints.
sqlite> select * from studies;
1|study|MINIMIZE
sqlite> ^D
$ ./bin/goptuna create-study --storage sqlite:///example.db --study study2
study2
$ sqlite3 example.db 
SQLite version 3.24.0 2018-06-04 14:10:15
Enter ".help" for usage hints.
sqlite> select * from studies;
1|study|MINIMIZE
2|study2|MINIMIZE
sqlite> ^D
$ ./bin/goptuna delete-study --storage sqlite:///example.db --study study
$ sqlite3 example.db 
SQLite version 3.24.0 2018-06-04 14:10:15
Enter ".help" for usage hints.
sqlite> select * from studies;
2|study2|MINIMIZE
sqlite>