Implement generic bucket

This bucket must be explicitly enabled and may be useful for users
wanting a generic caching mechanism that they can use directly instead
of via some predefined packaging system.

For instance, this enables one to implement a cache for packages
downloaded via wget or curl.

docs/buckets/generic.md

@@ -0,0 +1,41 @@
+# Generic
+
+This bucket is never enabled by default. You have to enable it explicitly like
+this:
+
+```ruby
+Vagrant.configure("2") do |config|
+  config.cache.enable :generic, { :cache_dir => "/var/cache/some" }
+end
+```
+
+The :cache_dir parameter is required. It specifies the directory on the guest
+that will be cached under the "generic" bucket.
+
+You may enable more than one generic bucket by giving them different names via
+the :name parameter, like this:
+
+```ruby
+Vagrant.configure("2") do |config|
+  config.cache.enable :generic, { :name => "one", :cache_dir => "/var/cache/one" }
+  config.cache.enable :generic, { :name => "two", :cache_dir => "/var/cache/two" }
+end
+```
+
+In this case you get two buckets called "generic-one" and "generic-two".
+
+The Generic bucket is useful if you want to implement a caching mechanism by
+hand. For instance, if you want to cache your wget downloads under
+/var/cache/wget you can do this:
+
+```ruby
+Vagrant.configure("2") do |config|
+  config.cache.enable :generic, { :name => wget, :cache_dir => "/var/cache/wget" }
+end
+```
+
+Then, you invoke wget like this:
+
+```sh
+wget -N -P /var/cache/wget URL
+```

lib/vagrant-cachier/bucket.rb

@@ -98,3 +98,4 @@ require_relative "bucket/apt_lists"
 require_relative "bucket/composer"
 require_relative "bucket/npm"
 require_relative "bucket/zypper"
+require_relative "bucket/generic"

lib/vagrant-cachier/bucket/generic.rb

@@ -0,0 +1,19 @@
+module VagrantPlugins
+  module Cachier
+    class Bucket
+      class Generic < Bucket
+        def self.capability
+          :generic_cache_dir
+        end
+
+        def install
+          if @configs.has_key?(:cache_dir)
+            @name = @configs.has_key?(:name) ? "generic-#{@configs[:name]}" : "generic"
+            symlink(@configs[:cache_dir])
+          else
+            @env[:ui].info I18n.t('vagrant_cachier.skipping_bucket', bucket: 'Generic')
+        end
+      end
+    end
+  end
+end