Add extension to filename automatically

[kohanyirobert]

Discussed in #251

Creating an issue/branch for a PR from the above mentioned discussion.

• The dropdown would appear only if in the settings one enables "override extension" or something
• The "override extension" setting is only active if one enables the "override filename" option
• When not enabled everything looks and works right now, if I specify a filename as "the filename" it'll downloaded as such (without extension)
• When enabled it defaults to something like "Auto" (in the dropdown), which would mean adding %(ext)s to the end of the --output template`
• Otherwise it can be prepopulated with popular options (mp4, mkv, mp3, webm)
• Optionally if the "override/select format" option is disabled it could populated with available options
• When I download something with the new option enabled and selected the "default" option it'd add the extension to the end of filename (e.g. 'the filenamebecomes 'the filename.mp4' if that's the extensionyt-dlp` determined suitable

(Tracking work in PR #255)