Emulate zsh in zlewidgets instead of static setopt command

This way the user can keep their preferred pushd syntax while enabling
us to use a standard syntax in our defined functions.

To explain further, without a clear value on the PUSHD_MINUS option, we
could be changing the +1/-0 values all we want, that some user would
find that it didn't work for him.

We have two options, then:
- Setting a particular value, which was my first approach.
- Using `emulate -L zsh` to ensure all options defined in the function's
  body won't be passed along to the main zsh instance.
  For more info see:
  http://zsh.sourceforge.net/Doc/Release/Shell-Builtin-Commands.html#index-emulate