[feature request] Option to load fixtures in bulk rather than row by row.

[leahrivkin]

The current implementation of dbfixture loads each fixture, then each row 1 by 1. This can get slow if you want to load large fixture files to have lots of data

Proposal: Update fixture YAML so you can run operations in bulk rather than individually

type fixtureConfig struct {
	Bulk bool `yaml:"bulk"`
}
type fixtureData struct {
	Model  string        `yaml:"model"`
	Config fixtureConfig `yaml:"config"`
	Rows   []row         `yaml:"rows"`
}