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