v2.0.0: Merge pull request #7 from getkalido/feature/rework

This migration library was in dire need of an update, to make it easier to maintain and use.

Changelog

• Functionality has been encapsulated, so that migrations can be managed for multiple databases without trampling each other's configuration.
• Migrations now need to be registered in a Registry object.
  • This does mean that the pattern of globally registering migrations in init() methods no longer work. The workaround is to define a Registry (by value, not pointer) in the package where migrations are generated. The zero-value of the registry is ready for use, so it doesn't need special initialisation.
  • The Registry should be thread-safe.
• Functionality is now accessed by running methods on a Migrator. This should be easier to use than emulating command-line arguments without proper type-checking.
• Case conversion now allocates fewer strings.
• Configuration is achieved by the options pattern. Any of the With*() functions can be called and passed to the Migrator's constructor as additional arguments.
• The package now defines a number of errors, which can be easily checked with errors.Is() from github.com/pkg/errors.