PR: Currency Symbol configurable.

Hi,

In Switzerland we do not use the Euro Symbol for our currency. It might be handy for others to configure the Symbol to 'CHF' or '$'.

<b>Attention</b>: as only in parameters.yml.dist configured it could break current installations.

Don't know if twig global is optimal and if it should be html entity or just the UTF-8 character €.

Greetings from Switzerland
Patrick