expand_path in file output

so that

```
output:
  file:
    directory: ~/foox
```

will work

1 files changed, 2 insertions, 2 deletions

diff --git a/lib/oxidized/output/file.rb b/lib/oxidized/output/file.rb
index bb13827..0b64447 100644
--- a/lib/oxidized/output/file.rb
+++ b/lib/oxidized/output/file.rb
@@ -17,7 +17,7 @@ class OxidizedFile < Output
   end
 
   def store node, outputs, opt={}
-    file = @cfg.directory
+    file = File.expand_path @cfg.directory
     if opt[:group]
       file = File.join File.dirname(file), opt[:group]
     end
@@ -28,7 +28,7 @@ class OxidizedFile < Output
   end
 
   def fetch node, group
-    cfg_dir = @cfg.directory
+    cfg_dir = File.expand_path @cfg.directory
     if group # group is explicitly defined by user
       IO.readlines File.join(cfg_dir, group, node)
     else