Add configuration option for user Vagrantfiles

This adds a config option, 'vagrantfiles', that takes an arbitrary list of relative or absolute paths to Vagrantfiles and merges them with the generated Vagrantfile. 

Internally, the list of Vagrantfile paths is converted to absolute paths and then to paths relative to the generated Vagrantfile.
@sethvargo I wasn't sure if you had a reason for checking the file's existence in your example. My thought is that the user should be informed if their Vagrantfiles are missing.