Add ability to change filename and/or filename template

[zzggbb]

Currently every video is downloaded in the format:

{youtube ID}-{youtube title}.{ext}

but it would be nice to be able to customize this a bit. I personally don't care to have the youtube ID in the filename, but it would be nice to see the resolution, like

{youtube title} [{resolution}].{ext}

Additionally, it would be nice to keep the last format as the default.

[AECX]

Good idea!