Restore method listing for foo(\t at REPL

This is somewhat intrusive, since we don't actually want to complete `foo(\t`, only show a list of completions. I'm not entirely sure this is the right approach, since users of REPLCompletions other than the REPL might want to do something special with the methods list, as IJulia does. I also fixed an extra newline at the end of the completion list if `length(completions) % num_cols == 0`.