Support for extra synced_folder options

Added ability to pass extra synced_folder options through new
config.cache.sync_opts -configuration.

README.md

@@ -27,6 +27,8 @@ Vagrant.configure("2") do |config|
     config.cache.auto_detect = true
     # If you are using VirtualBox, you might want to enable NFS for shared folders
     # config.cache.enable_nfs  = true
+    # You can pass extra mount options, for example:
+    # config.cache.sync_opts = {:create = true}
   end
 end
 ```

lib/vagrant-cachier/config.rb

@@ -1,7 +1,7 @@
 module VagrantPlugins
   module Cachier
     class Config < Vagrant.plugin(2, :config)
-      attr_accessor :scope, :auto_detect, :enable_nfs
+      attr_accessor :scope, :auto_detect, :enable_nfs, :sync_opts
       attr_reader   :buckets
 
       ALLOWED_SCOPES = %w( box machine )
@@ -10,6 +10,7 @@ module VagrantPlugins
         @scope       = UNSET_VALUE
         @auto_detect = UNSET_VALUE
         @enable_nfs  = UNSET_VALUE
+        @sync_opts   = UNSET_VALUE
       end
 
       def enable(bucket, opts = {})
@@ -34,6 +35,7 @@ module VagrantPlugins
         @scope       = :box  if @scope == UNSET_VALUE
         @auto_detect = false if @auto_detect == UNSET_VALUE
         @enable_nfs  = false if @enable_nfs == UNSET_VALUE
+        @sync_opts   = {}    if @sync_opts == UNSET_VALUE
         @buckets     = @buckets ? @buckets.dup : {}
       end
 

lib/vagrant-cachier/provision_ext.rb

@@ -18,6 +18,8 @@ module VagrantPlugins
             FileUtils.mkdir_p(cache_root.to_s) unless cache_root.exist?
 
             synced_folder_opts = {id: "vagrant-cache"}
+            synced_folder_opts.merge!(env[:machine].config.cache.sync_opts)
+            
             if env[:machine].config.cache.enable_nfs
               # REFACTOR: Drop the `nfs: true` argument once we drop support for Vagrant < 1.4
               synced_folder_opts.merge!({ nfs: true, type: 'nfs' })