Expand command list for more explicit functionality

[rleenders]

instead of relying solely on desired state migration specification which can get unclear at times (if I roll back to migration N, does that include rolling back N or keeping N applied, how to roll back all migrations including the 1st one, etc)

it would be smart to include explicit directional commands ie:
cassandra-migrate apply (to increment up one migration)
cassandra-migrate apply -a (to run all pending migrations)
cassandra-migrate apply -n ( to migrate up to and including a specified migration)

cassandra-migrate rollback (to rollback one migration)
cassandra-migrate rollback -a (to rollback all applied migrations)
cassandra-migrate rollback -n (to rollback down to and including the specified migration)

this should also simplify some of the parsing since we know explicitly which direction the user intends to go