This PR adds a new subcommand to validate migration files called validate.

Usage:
  pgroll validate <file> [flags]

Examples:
validate migrations/03_my_migration.yaml

Flags:
  -h, --help   help for validate

It is first step towards providing a way to validate migration files before running them. At the moment it can validate a single migration file against the latest schema in the database. This can be added to your local workflow, or a linter to make sure that the newly added migration file is correct.

Future improvements can be

• validating multiple migrations
• validating all unapplied migrations in a folder
• verifying up migrations on a subset of the table Allow for migration dry-runs to verify up SQL #169
• run jsonschema.Validate during validate Use JSON schema validation during Validate phase #673

The PR contains some minor changes to the migration phase Start. Now migration validation is decoupled from it. The previous implementation of Start started a new migration before validating the migration it got as a parameter. So this change does not interfere with the UX of pgroll.

Closes #370