Add indicate-current option into history command

[misebox]

This PR is aimed to add -i/--indicate-current option into history command.
This option indicates current version in a result of history command.
The requirement is Alembic version 0.9.9 or greater.

• with --help

$ FLASK_APP=app.py flask db history --help
Usage: flask db history [OPTIONS]

  List changeset scripts in chronological order.

Options:
  -d, --directory TEXT    migration script directory (default is "migrations")
  -r, --rev-range TEXT    Specify a revision range; format is [start]:[end]
  -v, --verbose           Use more verbose output
  -i, --indicate-current  Indicate current version (Alembic 0.9.9 or greater
                          is required)
  --help                  Show this message and exit.

• with --indicate-current

$ FLASK_APP=app.py flask db history -i
bc9874116395 -> 6dd1d5ec3dde (head), empty message
387f936c6387 -> bc9874116395 (current), empty message
<base> -> 387f936c6387, empty message

(current) is the sign for current version.

Please review or comment if you could.
And feel free for editing my code.
Thank you for reading.

[miguelgrinberg]

Thanks!