[WIP] Set some defaults early

Implements idea from issue #1664 

- [x] Create `runtime/defaults.vim` (definition of contents ongoing)
- [x] Load `runtime/defaults.vim` from `main.c:source_startup_scripts()`.
- [x] Update `runtime/doc/starting.txt`.

Notes: `defaults.vim` is mostly based on vim-sensible. I removed feature checks since they are unneded in neovim. 
Yeah, I placed it separatedly because of what was before discussed when I proposed to initialize the plugin hosts from the system nvimrc. I was left with the impression that it would be something that only distributors would want to touch, so the idea was to have defaults.vim be one layer below that. This way these defaults are guaranteed not to be messed up with by system admins or packagers. But whatever approach is fine.