Fix triple clicking empty line and dragging

[otomist]

Fixes core issue #4957. Adds a bool to SelectLine allowing the selectLine function to select lines that are empty. When starting a triple selection on an empty line the initial selectLine returns null because we don't see any characters, in this case we rerun selectLine but short circuit with the allow_empty_lines. We need to run selectLine with out allowing empty lines once because if there are characters on the line we don't want to select empty space.

[mitchellh]

The approach looks good, some requests.

[otomist]

Cleaned up the solution to use the pre-existing whitespace option in SelectLine. The logic in triple click drag hasn't changed. This way we don't add new unnecessary variables into screen.zip