allow for customising prefix of @attr names

[keithamus]

This is a feature we're intending to add just for the 1.x branch of Catalyst, in order to ease the transition into Catalyst 2.0.

This change will respect a attrPrefix static member on a class, and use that to derive the prefix of all @attrs in the class. This will default to data- meaning this is a non-breaking change, but for classes that wish to - they can add, for example, static attrPrefix = '' to remove the data- prefix. This allows us to transition controllers one-by-one while using Catalyst 1.x, which will make the transition to Catalyst 2.x much more straightforward.

This property will not be in Catalyst 2.0, as there is no need for it, as attrs are not prefixed with anything in Catalyst 2.0.