Add PreValidate hook (#359)

Fix #332.

This adds a new "PreValidate" hook that is executed before checking
the backup location. This allows, for example, mounting a remote
source to make the directories of the location available.

"PreValidate" is added as a new hook to avoid any breakage that might
have been caused by changing the behaviour of the "before" hook.

Documentataion updates included.

docs/pages/config.md

@@ -56,6 +56,8 @@ version: 2
 
 extras:
   hooks: &foo
+    prevalidate:
+      - echo "Wake up!"
     before:
       - echo "Hello"
     after: