drag and drop to select range

[PetterS]

I think drag and drop to select range would feel more natural than clicking first start, then end.

At least that was the first thing I tried, even after looking at the image in the readme.

[spiermar]

I believe this is a personal preference, but open to add it as an option. Open to PRs.

[brendangregg]

It might be possible to support both (still not sure we want to, but..). So if you click and release a block without exiting it (no mouse-out events), then it's the normal style. If you click and then mouse-out before the release, it's the drag style. Only catch is that you couldn't click and drag one block -- that would require a double click.