Make it possible to use a proc as archiver factory

[iconara]

Being limited to classes is inconvenient when you want to create your archiver with additional arguments.

Before you had to do something like this:

class MyArchiverFactory
  def initialize(some_arg)
    @some_arg = some_arg
  end

  def new(*archiver_args)
    MyArchiver.new(some_arg, *archiver_args)
  end
end

class MyArchiver < Jara::Releaser::Archiver
  def initialize(some_arg, *archiver_args)
    super(*archiver_args)
    @some_arg = some_arg
  end

  # …
end

Jara::Releaser.new(environment, bucket_name, archiver: MyArchiverFactory.new('foo'))

With this feature it can instead be done like this:

class MyArchiver < Jara::Releaser::Archiver
  def initialize(some_arg, *archiver_args)
    super(*archiver_args)
    @some_arg = some_arg
  end

  # …
end

Jara::Releaser.new(environment, bucket_name, archiver: proc { |*args| MyArchiver.new('foo', *args) })