Migrate: Add an option to assume "yes" to all prompts

[soroushj]

Is your feature request related to a problem? Please describe.
I want to run the migrate down command non-interactively in an automation script. But it prompts Are you sure you want to apply all down migrations? [y/N] and there's no option to bypass this.

Describe the solution you'd like
Add an option to the CLI, e.g. -yes, to assume "yes" to all prompts.

Describe alternatives you've considered
Running echo y | migrate down ....

[dhui]

use -all

This brings up a good point though. We should include help for sub-commands.

[dhui]

Created a new issue to track this: https://github.com/golang-migrate/migrate/issues/372