Restore correct indent behavior when inserting newlines

When explicitly auto-indenting and when pasting, indentation is based
on the previous non-blank line. When simply inserting newlines,
the previous line is used, even if it is blank.

Signed-off-by: Nathan Sobo <nathan@github.com>
Merge pull request #4231 from atom/mb-fix-newline-indent

Restore correct indent behavior when inserting newlines