[vim] Disable syntax checking by default
[vim] Use map & nmap instead of noremap & nnoremap

For consistency, only use `map` and `nmap` in `vimrc`
(they have the exact same effect as using `noremap` and `nnoremap`).

Ref: http://vim.wikia.com/wiki/Mapping_keys_in_Vim_-_Tutorial_%28Part_1%29