transparent notify
**Is your feature request related to a problem? Please describe.**

In current version of spacevim, the `notify` api support pop-up notification. now request for transparent notify.

**Describe the solution you'd like**

```vim
let s:N = SpaceVim#api#import('notify')
call s:N.notify('hello world', { 'color' : 'WarningMsg', 'stype' : 'transparent'})
```


ref links:

- https://github.com/neovim/neovim/pull/9923
- https://github.com/neovim/neovim/issues/7064