Add some literal namespaces

[ToruNiina]

Hi,

First of all, thank you very much for developing this plugin! I am really enjoying this.

I found that std::literals::string_view_literals is not highlighted. Since string_literals is highlighted as a namespace, I concluded that it is just overlooked. I added string_view_literals and another overlooked namespace, complex_literals introduced in C++14, and I'm sending this PullReq.

Could you check it? Thank you.