adding merge_direct_overlaps() method

[Ramalh]

Greetings dear developers,

I have been using intervaltree for my project which is about biology.
But when I use merge_overlaps() method, it merges all intervals that overlap directly or indirectly.
Assume there are three regions in a genome called intervalA, intervalB, and intervalC. intervalB overlaps with both
intervalA and intervalC but these two are not.
So what I needed to merged them like this: intervalAB and intervalBC. And I wrote a method called merge_direct_overlaps(). I have tested it with four additional test function for mentioned python version and it passed all.

I tried to make the insertion as smooth as possible; such as copying the structure of merge_overlaps() method for this new method, adding 4 test functions which is highly similar to test functions of merge_overlaps(), and lastly adding a brief info about the method to README.md file.

Feedback is welcome