[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. 
> We could change the actual internal definition of `<c-l>` to clear `hlsearch`, such that this mapping is not necessary; but then that could break some other plugin. I can't imagine what it would break, so maybe it is acceptable for this case.

AFAIK this will only change `v:hlsearch` variable (which is rather recent addition) and `screenattr()` (which is intended for testing only).