Add init optionality

# Current behavior

If db-migrate tries to connect to a database that does not yet exists it is going to throw an error. The user needs to create the database with fitting permissions first.
# Proposed solution

Add a command line option `--init` which tells db-migrate to first check if the database exists and then tries to execute a database execution. This tasks has items listed below.
# Affected drivers

All: New method `showDatabases` is needed

Items:
- https://github.com/db-migrate/pg/issues/6
- https://github.com/db-migrate/mysql/issues/2
- https://github.com/db-migrate/db-migrate-base/issues/2
# API extensions
- New command line option
##

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