Use webm extension by default when --webm is specified

[tdaede]

--webm seems to create a properly tagged WebM file, but the default output filename is still .mkv.

[master-of-zen]

@tdaede I took different approach, removed --webm flag. .mkv is the default extension but in -o you can specify mp4 or webm (it's required to add check for container compatibility before encoding in future, atm even if it fails at contat, you can change output file to another container and --resume and it will work), it should also take care of another issue https://github.com/master-of-zen/Av1an/issues/322