CLI-driven database management

[florimondmanca]

Related to #4

I really like how Django allows you to manage databases through its management command system. Ideally, I would like databases to be managed through the Bocadillo CLI, with commands such as:

# Initialize the Bocadillo app for databases, creating files or asking for information as necessary
boca init:db [<backend: (sqlite|postgres|mysql)>]

# Apply all unapplied migrations
boca migrate

# Apply a specific migration (perhaps an older one, meaning rolling back)
boca migrate <migration_number>

# Generate a migration script
boca migrations:create [<name>] [--table <table>]

# See applied/unapplied migrations
boca migrations:show

The Orator ORM provides similar management commands, so perhaps it could be an inspiration.