Better interface for slate-bind-copy-paste

[zhujinxuan]

1. We shall have function merge(first, second)
2. We shall split rules into mergeFront, mergeEnd, insertBetween and use (range, fragment) as return; and normalize the range between rules.

[oyeanuj]

@zhujinxuan Thanks for creating these plugins! For the copy-paste, could you also add some documentation on the benefits or the use-case for the plugins? It would really help people assess if it is something they need to have as well :)

[zhujinxuan]

@oyeanuj I think I will do it in next month. I am busy on something else currently.

[zhujinxuan]

@oyeanuj Sorry. I think I will do it in Dec. I do not have time on it recently.