Add documentation for hooks option

[daun]

Description

Document new hooks option for registering handlers in constructor:

const swup = new Swup({
  hooks: {
    'visit:start': () => console.log('starting visit'),
    'page:load': () => console.log('loaded page'),
    'visit:end': () => console.log('finished visit')
  }
})

Checks

• The PR is submitted to the master branch
• The code was linted before pushing (npm run lint)
• All tests are passing (npm run test)
• New or updated tests are included
• The documentation was updated as required

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