Remove `Shellwords.join()` and keep ARGV as-is

As per the discussion in #5 this PR removes the shell escaping via `Shellwords.join()` completely rather than trying to make it work on windows.

Why?
* vagrant-wrapper is just a proxy / middle-man
* whatever comes in to vagrant-wrapper should be passed unmodified to vagrant
* if something needs to be shell-escaped that's in the responsibility of the caller, not us

This *might (potentially)* affect existing linux users, but it should not be a problem:
1. this behaviour is nowhere documented (i.e. not public API) users should not rely on it
2. if something breaks this indicates that callers (probably unknowingly) missed to escape the input properly

/cc
@btm for you to review / test
@binarybabel if @btm agrees we can close #5 in favor of this PR