Allow passing options to source elements within video/audio tags.

Previously, in order to add type or media attributes to source tags you
would have had to construct the video/audio tag yourself. Now you can just
pass an array of hashes with the options you want to use.

Example:
    video_tag([{ src: 'trailer.ogg', type: 'video/ogg'}])
    # => <video><source src="/videos/trailer.ogg" type="video/ogg" /></video>