Add support for caching pip packages

These directories are chached for pip packages:
$HOME/.cache/pip/http
$HOME/.cache/pip/wheels

lib/vagrant-cachier/bucket.rb

@@ -101,3 +101,4 @@ def symlink?(path)
 require_relative "bucket/npm"
 require_relative "bucket/zypper"
 require_relative "bucket/generic"
+require_relative "bucket/pip"

lib/vagrant-cachier/bucket/pip.rb

@@ -0,0 +1,22 @@
+module VagrantPlugins
+  module Cachier
+    class Bucket
+      class Pip < Bucket
+        def self.capability
+          :pip_cache_dir
+        end
+
+        def install
+          if guest.capability?(:pip_cache_dir)
+            if guest_path = guest.capability(:pip_cache_dir)
+              symlink("#{guest_path}/http", "/tmp/vagrant-cache/#{@name}/http")
+              symlink("#{guest_path}/wheels", "/tmp/vagrant-cache/#{@name}/wheels")
+            end
+          else
+            @env[:ui].info I18n.t('vagrant_cachier.skipping_bucket', bucket: 'pip')
+          end
+        end
+      end
+    end
+  end
+end

lib/vagrant-cachier/cap/linux/pip_cache_dir.rb

@@ -0,0 +1,20 @@
+module VagrantPlugins
+  module Cachier
+    module Cap
+      module Linux
+        module PipCacheDir
+          def self.pip_cache_dir(machine)
+            pip_cache_dir = nil
+            machine.communicate.tap do |comm|
+              return unless comm.test('which pip')
+              comm.execute 'echo $HOME' do |buffer, output|
+                pip_cache_dir = output.chomp if buffer == :stdout
+              end
+            end
+            return "#{pip_cache_dir}/.cache/pip"
+          end
+        end
+      end
+    end
+  end
+end

lib/vagrant-cachier/capabilities.rb

@@ -36,6 +36,11 @@ class Plugin < Vagrant.plugin('2')
         Cap::Linux::NpmCacheDir
       end
 
+      guest_capability 'linux', 'pip_cache_dir' do
+        require_relative 'cap/linux/pip_cache_dir'
+        Cap::Linux::PipCacheDir
+      end
+
       guest_capability 'debian', 'apt_cache_dir' do
         require_relative 'cap/debian/apt_cache_dir'
         Cap::Debian::AptCacheDir