add seeking support for creating thumbnails of video

[attenzione]

support time in style params, including lambda

model Fake

  has_attached_file :attachment, url:  '/uploads/:class/:id/:style.:extension',
                                 path: ':rails_root/public:url',
                                 styles: {
                                    thumb: { format: 'jpg', time: -> (attachment) { attachment.instance.method(:style_time_for) }, processors: [ :transcoder ] }
                                 }

  def style_time_for(meta, options)
    meta[:duration] / 2
  end

end

should fix issue #2

[attenzione]

Travis CI build failed because it's required updated av gem, which i already submitted. Need to bump version to 0.8.2 and resubmit this request with fixed dependencies

[owahab]

AV 0.9.0 was released. Please update the pull request.

[coveralls]

Coverage increased (+0.48%) to 92.68% when pulling a2463ae on attenzione:master into b8f777e on ruby-av:master.

[tux-mind]

please explain this feature into the README too 😊