Fix fish subcommand completion
Fix fish subcommand completion

This allows subcommand style plugins to properly autocomplete.
Existing commands are not affected. 

Example, say you have support for `rbenv foo bar --flag`, then
this allows the last `--flag` argument to be properly completed.
cehoffman/rbenv,nodenv/nodenv,viclim/rbenv,rbenv/rbenv,rbenv/rbenv,jasonkarns/rbenv,viclim/rbenv,docwhat/rbenv,sebhahn/pyenv,cehoffman/rbenv,jrolfs/nodenv,OiNutter/nodenv,yyyc514/rbenv,yyyc514/rbenv,yyuu/pyenv,sebhahn/pyenv,cehoffman/rbenv,jasonkarns/rbenv,sstephenson/rbenv,nodenv/nodenv,OiNutter/nodenv,yyuu/pyenv,pyenv/pyenv,pyenv/pyenv,viclim/rbenv,sebhahn/pyenv,pyenv/pyenv,docwhat/rbenv,sstephenson/rbenv,yyyc514/rbenv,jasonkarns/rbenv,sstephenson/rbenv,docwhat/rbenv,jrolfs/nodenv