fallback to Rg if available

- Category
- [ ] Question
- [ ] Bug
- [x ] Suggestion
- OS
- [x ] Linux
- [x ] macOS
- [x ] Windows
- [ x] Etc.
- Vim
- [x ] Vim
- [x ] Neovim

Rg and Ag are very similar (there are other alternatives, such as Ack or even plain grep that can be used for the same purpose). I've recently switched to Rg from Ag to get some additional speed. I have written a custom Rg command for fzf.vim and works great.

However I would prefer to avoid customizing fzf.vim and instead modify fzf.vim to fallback to use rg if ag is not available (or perhaps expose an option to choose which binary to use). I would be happy to contribute a pull-request for this, just want to make sure it has a chance of being included.