Add --skip-validation flag to start command

[andrew-farries]

Add a --skip-validation flag to the start command.

If set, migration validation is skipped.

For example, this table has a NOT NULL name field:

{
  "name": "01_create_table",
  "operations": [
    {
      "create_table": {
        "name": "products",
        "columns": [
          {
            "name": "id",
            "type": "serial",
            "pk": true
          },
          {
            "name": "name",
            "type": "varchar(255)",
            "nullable": false
          }
        ]
      }
    }
  ]
}

starting this migration will fail as the name field is already NOT NULL:

{
  "name": "02_set_nullable",
  "operations": [
    {
      "alter_column": {
        "table": "products",
        "column": "name",
        "nullable": false,
        "up": "name",
        "down": "name"
      }
    }
  ]
}

Error: migration is invalid: column "name" on table "products" is NOT NULL

But if the start command is invoked with --skip-validation then the start command succeeds.

This is part of #239