https://github.com/vim/vim/pull/8544 introduced a change that displays an error when `nargs` is not defined and/or defined incorrectly. As a user, this will display an error message each time upon starting up. The error shows up as
```
Error detected while processing ~/.vim/plugged/vim-ripgrep/plugin/vim-ripgrep.vim:
line 149: E1208: -complete used without -nargs
Press ENTER or type command to continue
```
This fix to add `-nargs=?` to each of the `command`s.
Fixes#57
- Only wrap the search string in quotes during visual search. As was the
behavior prior to 7d01d697.
- Restore the ability to use all ripgrep features from command mode:
`:Rg --fixed-strings \.Model service/`
fixes#25
