More useful split-selection-into-lines

Changes `split-selection-into-lines` to work the same way as Sublime Text.

Fixes #1704.

Test Plan:

  * Ran `script/build`
  * Opened a new instance of Atom
  * Selected some lines via the gutter
  * Executed `split-selection-into-lines`
  * Saw one selection for each line that was visually highlighted
  * Selected some lines with the mouse, including a partial one at the end
  * Executed `split-selection-into-lines`
  * Saw one selection for each line that was visually highlighted