[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. 
>  I was left with the impression that it would be something that only distributors would want to touch

I thought about that, but who suggested that we can't distribute a "recommended nvimrc"? 

In fact I think the practice of distros modifying the system nvimrc should be stopped in most cases. In most cases the distro is trying to correct Vim's own inertia, whereas with Neovim we want to solve this upstream. We want a consistent experience for all users.

I would like to hear opinions from package maintainers though.