[Translation] Make placeholder prefix and suffix configurable

[hruben]

JSFV uses the same error messages as the Symfony constraints. So it appears only natural to me to use the same placeholder marks. However, jsfv.getComputeMessage() always sets these marks to '{{ ' and ' }}'.
It would save a lot of effort to have some easy way of changing this back to the ExposeTranslationBundle's default value of '%'. One option is to make the placeholder prefix and suffix a parameter in parameters.yml. Another option is to add a block around jsfv.getComputeMessage() so that it can be easily overridden.

[recipe]

@Abhoryo why do we use braces? I agree with @hruben we need to use "%" by default.

[Abhoryo]

Braces were the ExposeTranslationBundle's default value.

[hruben]

In 0.3.x-dev, it's '%', see vendor/willdurand/expose-translation-bundle/Bazinga/ExposeTranslationBundle/Resources/public/js/translator.js line 335. And even if it was the default value, I would like to be able to change it. The intention is to use the same translations for PHP and JS generated messages, so '%' surely makes sense.

[Abhoryo]

You can change the default for the moment.