fallback clang options when no configuration found

[unnamed42]

It's quite unconvenient to get no c++11 support when editing a standalone cpp file without specifically telling clang to turn it on (for example, no emplace_back in std::vector's completion candidates). I think it may be better to add a default compiler option to feed compilers, just like what we do in ale:

let g:ale_cpp_clang_options = '-std=c++14 -Wall -Wextra'

My setup: vim 8.0.1415 https://github.com/roxma/nvim-completion-manager/commit/e724a442072261993ca503e969d2cb25722ab1d2 https://github.com/roxma/vim-hug-neovim-rpc/commit/60093847f0ba0a57ace54df30bd17a8239a99d6f https://github.com/roxma/ncm-clang/commit/2cd08c144742dfde8ce6d2914da1cec83c9d1513