Web component compatibility

[antoine92190]

When using your library inside another library (such as mine: https://github.com/antoine92190/vue-advanced-chat) that will be used in any project as a web component, the line below is throwing an error:

vue-infinite-loading/src/components/InfiniteLoading.vue

Lines 318 to 320 in 4baed2b

	if (typeof this.forceUseInfiniteWrapper === 'string') {
	result = document.querySelector(this.forceUseInfiniteWrapper);
	}

I managed to fix the issue by doing this instead:

if (typeof this.forceUseInfiniteWrapper === 'string') {
  const component = document.querySelector('vue-advanced-chat');
  if (component) {
    result = component.shadowRoot.querySelector(this.forceUseInfiniteWrapper);
  } else {
    result = document.querySelector(this.forceUseInfiniteWrapper);
  }
}

The component const could be a prop that we are passing to vue-infinite-loading.
Please let me know if you are interested by this fix, I will be happy to make a PR for that.