feat: add smooth scroll when clicking on a menu item

[kimyvgy]

This will close #9 by adding options for smooth scrolling feature. For example:

1. Using via CDN:

<script src="/path/to/dist/simple-scrollspy.min.js"></script>
<script>
  window.onload = function () {
    scrollSpy('#main-menu', {
      sectionClass: '.scrollspy',
      menuActiveTarget: '.menu-item',
      offset: 100,
      smoothScroll: true,
    })
  }
</script>

2. Using with NPM package:

import jumpTo from 'jump.js'

scrollSpy('#main-menu', {
  // ....,

  // enable smooth scroll:
  // - true: enable with the default scroll behavior
  // - false: disable this feature
  // - object: enable with some options that will pass to `window.scroll` or `smoothScrollBehavior`
  //   + Default behavior: https://developer.mozilla.org/en-US/docs/Web/API/Window/scroll
  //   + Jump.js: https://www.npmjs.com/package/jump.js
  smoothScroll: {
    duration: 1000, // only works with jump.js,
    offset: -100,   // only works with jump.js,
  },

  // customize scroll behavior,
  // - default: window.scroll({ behavior: 'smooth', ...smoothScroll })
  // - customize: you can define your scroll behavior. Ex: use `jump.js`, jQuery, .etc
  smoothScrollBehavior: function(element, options) {
    // use `jump.js` instead of the default scroll behavior
    jumpTo(element, options)
  }
})

[kimyvgy]