Change how peco.Finish handles the line under cursor

Only add the line where the cursor is only if there are no other
selected lines before hand. This apparently matches what percol does,
and I agree that it's more intuitive

refs: http://twitter.com/mattn_jp/status/485951916040413184
Merge pull request #149 from peco/add_current_line_only_if_no_selection

Change how peco.Finish handles the line under cursor