R1:ceb522427ed7

Highlight lines on drag and drop in Diffusion

Summary: Nobody knows that it's possible to highlight more lines because there's no interface for it.

Test Plan:
Highlighted:

- single line
- top to bottom
- bottom to top
- inside to outside

Reviewers: Korvin, epriestley

Reviewed By: epriestley

CC: aran, epriestley

Differential Revision: https://secure.phabricator.com/D3150