fix : show short_file_name properly in Windows

[Jaehaks]

/// Problem: #32

1. Set configuration doesn't be applied in M.toggle_quick_menu() It makes short_file_name option don't work
2. The number of slash('/') doesn't show properly in short_file_name mode. On Windows, sometimes the file path has both '/' and '\' as delimiter because of neovim inherit problem.

/// Solution:

1. call get_config() function in toggle_quick_menu again. 2) On Windows, backslash of file path in buffer list is replaced by '/'

[j-morano]

Hello. What is the status of this pull request? Does it introduce new functionality or just fix this issue for Windows?

[Jaehaks]

@j-morano This PR suggests dealing with short_file_names for Windows issue as you see... short_file_names still doesn't work although 03df014 is commited.

toggle_quick_menu() show the relative path only when new buffer is added without the number of slash. It remains showing absolute paths whenever I restart neovim and call the session opend before. I think initializing of config has problem when execute toggle_quick_menu(), but it is weird because I don't know why the configuration cannot save as other plugins.