[Feature Request] loadmore can't pull up at bottom on mobile phone

### Existing Component
是

### Component Name
loadmore

### Description
When swiping up the list, loadmore will excute function `checkBottomReached`. If it returns false, the bottom loading area will not be activated.
```js
// checkBottomReached
return this.$el.getBoundingClientRect().bottom <= this.scrollEventTarget.getBoundingClientRect().bottom + 1;
```
Loadmore works well on my computer. But when I test it on a mobile phone, I can't pull up further when the list has actually reached the bottom. Then I use `alert` to display the bottom value, and I got 464.89 for `this.$el`, and 463.4 for `this.scrollEventTarget`. So `checkBottomReached` will return false.

Some little change would help fix this.
```js
return this.$el.getBoundingClientRect().bottom <= this.scrollEventTarget.getBoundingClientRect().bottom + 2;
```