`merge_neighbors()` does not work with `datetime` objects

[murrple-1]

Due to this line:

intervaltree/intervaltree/intervaltree.py

Line 827 in 6228bb1

margin = higher.begin - lower.end

__sub__() is not defined for dateime.datetime, dateime.time or dateime.date objects, though they work in most (all?) other contexts, and seem to be "supported".

[murrple-1]

Proposing that the function have an optional parameter like:

...
margin_fn=lambda begin, end: begin - end,
...