`preview.rb` parses path incorrectly if it contains special characters (e.g. `+`)

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

Calling `:call fzf#vim#ag("", fzf#vim#with_preview('up:60%'))` and trying to preview a file with a `+` in the path will display nothing in the preview window.
The problem is the `COMMAND.gsub` in `preview.rb` line 20: it escapes `my/path+b/file.txt` to `my/path\+b/file.txt`, which gets picked up by `gsub` as a capture group and gets removed, so that the path looks like this:
`my/pathb/file.txt`.

=> This file can't be read, nothing is displayed.

Quick fixing hack was to change the escaping to something like `COMMAND.gsub('{}', Shellwords.shellescape(Shellwords.shellescape(path)).gsub('\\\\', '\\'))`, obviously this could be improved in some form.