[Parser] Added Configuration to globally enable or disable parsers

Hey There,

because of the Problems with #295 and #291 i have implemented the Feature to disable Parsers. It  is possible to deactivate the Parsers per Action but that is a pain in the ass if you want to disable them on every action. So if this is merged you can disable them in the config like:

``` yml
# app/config.yml
nelmio_api_doc:
    parsers:
        ValidationParser: false
```

All existing Parsers (at state of this PR) are enabled by default
I don't think adding this getter makes sense