Provider plugins can provide arbitrary options
UI objects use "scopes" now
Make the Basic/Colored UI objects threadsafe
BatchAction for running actions in parallel
Run "up" in a batch action so it parallelizes
Environment#batch and forcing disable through env var
Up command now uses Environment#batch
Allow only one Environment#batch to run at a time per env
Gracefully handle errors in parallel cases
Lock around chdir in Subprocess for thread safety
Re-disable parallel on virtualbox
SafeChdir all the things for thread safety
Merge branch 'parallelize-providers' into 1-2-incoming

Providers will now `vagrant up` in parallel! This can easily be extended
to other commands as well.

Note that the provider must explicitly support parallelization for this
mode to take effect. VirtualBox does not yet support this, for example.

The core of Vagrant should be thread-safe at this point, as well.