Paths of completion definitions in this repo are inconsequential, and I
don't know a convention for that, so I just modelled it on git.git
https://github.com/git/git/tree/master/contrib/completion. What's
different here is that instead of using a uniform naming scheme for each
file (you-get-completion.<shellname>), I'm using the conventional name
for each shell, which means _you-get for Zsh, for instance. Users of
each shell should be able to pick out the completion definition for
their shell based on the filenames; in the worst case, they can still
tell from the comment at the top of each file.
Note that I'm no expert on Bash or Fish completion, so
you-get-completion.bash and you-get.fish may not be written in the
idiomatic way. (you-get-completion.bash is most certainly not idiomatic,
if there's an idiom at all.)
Closes#1178.
