Stop inserspersing stream output with blank lines.

Previously, the output would be interspersed with blank lines when
piping input to ag, i.e. `echo "a\nb" | ag .` would output `a\n\nb\n`
instead of `a\nb`. This commit fixes this issue.
Merge pull request #430 from aeriksson/remove-empty-lines-for-streams

Stop interspersing stream output with blank lines.