slug fails when property not found in special pages

I have at least 2 scenarios where property is not defined and therefore causes slug to fail:

1. An index.md file which redirects from / -> /blog
2. Listing page

These source files are like below:

```
---                                                                                                                                        │
template: index.jade                                                                                                                       │
---
```

So I made 2 changes:

1. Created a wrapper method which takes renaming as parameter and terminates if property not defined
2. Terminates renaming when input filename equals to output filename (it deletes the file if not terminated)

Hope you'll find these changes appropriate. Thanks for the plugin!