[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, it seems like I tested this in vim instead of nvim. In nvim the behavior is different, and the default seems OK (as OK as having nvim yielding to the terminal is, but I'm not sure if that's a problem on my end or a quirk in nvims implementation of `:!`). Either way, I'll remove this.