[DX] generate:crud minor improvements

This relates to [issue 283](https://github.com/sensiolabs/SensioGeneratorBundle/issues/283). Rename $entity to something real - like $product or $blogPost for a BlogPost entity
Ok, so do you have any suggestions? Some options I've thought of are:
a) include a check to see if the entity name ends in either "s" or "es" already. If it does, use "entities" but if not, then pluralise it.
b) Have a predefined list of common entity names eg post, article, user etc. which can be plurarlised but if the entity name isn't in the list use the generic "entities" instead.
c) a combination of both a) and b)
d) leave the generic "entities" when describing multiple entities but use "real" names for description of single entities.