Passing output_filename to fuel_convert

[sygi]

When you convert a dataset, fuel_convert creates a hdf5 file of some default name. I saw the converters have the option output_filename to overwrite default behavior, but one cannot pass it to fuel_convert.
It would be nice to have, especially for youtube audio, where one currently does:

data = YouTubeAudio('XqaJ2Ol5cC4')

but it could be:

fuel-convert youtube_audio --youtube-id XqaJ2Ol5cC4 --output_filename music
data = YouTubeAudio('music')

I have written it for my usage - I'll send you a PR and you'll be able to merge it if you'd like to.