[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. 
Actually, since evim is going away, I don't think we need defaults.vim. We can put the defaults in the system `nvimrc`. We need to add that to `runtime/` since it does not appear to exist.