A key frustration with the prior version of mkvextract completions was
that even in a position where a filename would be expected, no
completions for a filename were offered. This update introduces more
rigorous argument handling, most importantly restricting
track/attachment completion to when both a mode and a file are
specified.
* add completions for mkvextract
* fix edge cases with option placement in mkvextract.fish
* improve resiliency to errors in mkvextract.fish
* minor fixes in mkvextract.fish
