Turn off conceal completely if requested

It can be confusing if `g:vim_json_syntax_conceal` is set to `0`
but `conceal` is set to a non-zero value (e.g. `1` in my case) by
the user's `.vimrc`.

This sets `conceal` to `0` if `g:vim_json_syntax_conceal` is set
to `0`.