Support for extra synced_folder options

Added ability to pass extra synced_folder options through new
config.cache.sync_opts -configuration.
Merge branch 'master' of https://github.com/eaapa/vagrant-cachier into next
Update docs to reflect what's going to be implemented
Replace `enable_nfs` and `sync_opts` config with `synced_folder_opts`
Mark `config.enable_nfs` as deprecated
Mention `synced_folder_opts` on CHANGELOG
Lock vagrant to 1.4.3
Display a warning using Vagrant's UI in case enable_nfs is set
Display enable_nfs deprecation warning only once
When specifying NFS synced folders we need to use a symbol [GH-73]