Add skip command to support versioning pre-existing db's

[stephen-kruger]

Add support for a "skip" command which will process all .sql migration files as no-operations, but still write the schema table version with the last found version.

This provides a way to apply sql-migrate versioning to pre-existing databases where you don't necessarily want to run from the initial schema file, but from some later point.

Supports all the same flags and options as the Up command.

[rubenv]

Cool, that makes sense.

Could you delete the commented (unneeded) code?

[stephen-kruger]

Absolutely, apologies for leaving it in - I'll lean the code up and commit a final version.

[rubenv]

Looks excellent, thanks a lot!

I hope sql-migrate serves well @grab, happy to make things easier!