*Suggestion* use function for defining versions

[Alexxfrolov]

Suggestion

Use constant or functions to generate versions (same as for acl, you can define it via @acl or via function def acl(:thumb) )

Reasoning

In some cases anyone would like not to generate full number of file versions. Right now you can skip transformations via

def transform(version, scope), do: :skip

Maybe it would be more native defining it in def versions. So you don't have to define empty transformations

Example

@versions [:original, :thumb]

or use

def versions({file, scope}) do
  [:original, :thumb]
end