drag and drop to select range about flamescope

[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.