Allow for custom provisioning to replace the default

It's possible that the framework and directory structure we are providing in Varying Vagrant Vagrants could be useful in the wild without the default provisioning. By allowing for a custom provisioning script, we can use the same project structure with little ramp up time on different provisioning setups.
Add pre/post provisioning hooks to Vagrantfile

If provision-pre.sh or provision-post.sh exist, they are used for additional provisioning with either provision.sh or provision-custom.sh
Introduce a flags system to disable select portions of default provisioning

Allows for simple files to be created in the `provision/flags/` directory that trigger the avoidance of other shell scripting when the provisioning is running.
Merge pull request #51 from 10up/provision-hooks

Pre/Post Provision Hooks, Custom Provision Replacement, Provisioning Flags