Migration backends

[sastraxi]

Right now, we only support knex for migrations. However, there are many different migration tools out there for different stacks:

• Flyway
• django
• rake

We can support these by adding a bit of logic around swapping out subcommands based on a new configuration parameter in .pgshrc: migrations.backend.

• by default, this config value will be unset
• we'll move the src/cmd/knex directory to src/cmd/migrate/knex
• we'll use the subcommand implementation in the subdirectory that corresponds to the configured backend
• each backend will implement the up, down, force-up, force-down, and validate subcommands
• each backend will implement a detect function that returns a truthy value iff evidence that this migration tool is used in the current project directory
• if a migration subcommand is run and no backend is set:
  • each detect command will be run, and the first backend to detect itself becomes the configured backend (with a corresponding message to the user and write to .pgshrc, if one exists)
  • if no backends are detected, the command will exit with an error

[sastraxi]

Initial implementation sketch in #47