Add additional_options parameter to set any ckeditor parameter

I've added an additional_options parameter to the configuration which allows an array of options to be set for CKEditor. This will allow any ckeditor option to be passed to the configuration file without having to be supported by the CkeditorBundle already.

One tricky part is that the value of the options needs to evaluate to be valid javascript, so booleans need to be strings, and strings need to be pre-quoted. For instance:

```yaml
trsteel_ckeditor:
    additional_options:
       enterMode: CKEDITOR.ENTER_BR
       option1: "true"
       option2: 12345
       option3: "'test'"
```