Add an option to define the output directory for cobertura xml reports.

The output directory can now be defined via property `output_directory` in the `.slather.yml` file or the commandline option `--output-directory`.

Relates to issue #35 

Regards
tarbrain.
Can we use `File.join` instead:

```ruby
File.join('a', 'b')
=> "a/b"
```