Allow falling back to mkv container when merging incompatible video & audio

As explained in https://github.com/rg3/youtube-dl/issues/3610, there are cases when downloading separate video and audio and then merging when the video container (usually webm containing VP8/VP9) is incompatible with the audio codec (aac/mp3). By default, youtube-dl attempts to create a merge in the container format of the video file. 

In this pull request I've attempted to add a workaround option "--fallback-mkv" that, when merging the two files, attempts to merge in the normal way first, and if that fails due to codec incompatibilities, attempts to merge to mkv container output instead.