feat!: Add pipe mode

[rekmarks]

Closes #17

Adds a pipe mode to the CLI, enabling it to be used as, well, a pipe. A --file mode may be added in the future, but such a mode can now also be emulated in userland.

Changes

• Add serialize()/deserialize() methods to Migration
• Add pipe mode to CLI
  • This mode reads from stdin and writes to stdout
• Make credential validation more stringent
  • This should catch errors at an earlier stage.
• BREAKING: Refactor main module exports

TODO:

• Docs / example usage
• Allow passing in just credentials to start a migration