distinguish "toBottom" and 'toBottom with bottom-threshold"，"totop" and 'toBottom with top-threshold"

[masongzhi]

What kind of this PR? (check at least one)

• [ ] Bugfix
• [x] Feature
• [ ] Code style update
• [ ] Build-related changes
• [ ] Other, please describe:

Other information: when i want to use toBottom for css style，and want to set bottom-threshold to load more data；and then this two case use same event. toBottom，it puzzled me;

因为ios会有弹性滚动的功能，如果bottom-threshold是0，则一直滚动是不会处罚toBottom的，所以我加了bottom-threshold为50，但是这又会影响我css触发到底部的样式，所以我拆分成2个event