o with no arguments opens current directory

Replace `o` and `oo` open aliases with multi-purpose function. `o` with no arguments just opens the current directory. If any arguments are given, then that location will be used instead. I realize this is a big change and a little bit of a new way of thinking but I have been using this for a while now and it's quite nice. Saving keystrokes here. Got this idea from [@thoughbot/dotfiles](https://github.com/thoughtbot/dotfiles) `g` function.