Adding video/audio download support

Fixes https://github.com/bbolli/tumblr-utils/issues/32 :

- Audio files hosted on Tumblr and Soundcloud are now downloaded and
embedded into an HTML5 audio tag.
- Video files hosted on Tumblr are downloaded and embedded into a video
tag.
- External video files can be downloaded if Youtube-DL is installed on
the system.
- The option -k now skips all medias download.
- The new option -K skips external videos download.
- The new option -M skips *external* videos which have a size over the
specified limit.

I switched the images folder to a 'media' folder. But I now realise it may be stupid for backward compatibility.
Let me now what you think of this PR, and I'll provide fixes accordingly.