[vim] Use backslash for Windows filepaths (#896)

- Fix shellescaping issues for filepaths
    - Supports both forward slashes or backslashes
    - Paths with spaces
- Use jobstart for neovim in s:execute (Windows)
    - https://github.com/neovim/neovim/pull/6497
- Make 2 s:fzf_shellescape functions
    - (Windows) Substitute \" with \\" to escape the last backslash
    - (Default) Regular shellescape
- Support list 'options'
- Add "@Echo off" to the batchfile used to execute fzf