Use 'swap' strategy in inline-critical

[kyrylkov]

Addresses #581

[bezoerb]

the strategy can already be passed as option to inline-critical:

generate({
  ...
  inline: {
    strategy: 'swap
  }
});

or using the cli:

critical my.url.com --inline-strategy=swap

[bezoerb]

This prevents the strategy from being set via options