Allow adding custom commands to the generated Vagrantfile

[mbautin]

No description provided.

[sethvargo]

After discussion, we think these patterns could quickly grow out of scale. Instead of handling all the special edge cases, I've created #51 to allow users to specify a full Vagrantfile which can be fully customized.

[fnichol]

If you still require this and can't get it out-of-the-box with this driver, now you can specify your own custom Vagrantfile ERB template which will be used instead as of cc75795. Thanks!