Integrate shell completions into kbsecret script

[woodruffw]

Maintaining separate files for each shell completion syntax is a chore, and requires users to fetch the files from the repo (RubyGems doesn't provide a way to expose shell completion for a script).

So, it'd be nice if we could do this instead:

# .bashrc or .zshrc or whatever fish uses
# use $SHELL to dump the correct shell's completions
eval "$(kbsecret --completions)"