Include timestamp and uploader as part of file name

[arpith20]

Including the timestamp and uploader (channel name) in name of the file makes it easier to find the required video easily.

[seler]

In my optinion its better to allow user to specify the filename format himself - YoutubeDL library allows that. I've created a pull request #8 where you can specify output template that gets passed to youtube-dl.
Example use is:

/usr/local/bin/youtube-dl-subscriptions --output "/home/user/yt/%(timestamp)s-%(uploader)s-%(title)s-%(id)s.%(ext)s"

You can read more about formatting output in youtube-dl docs.