Fix C++11 raw string delimiter

Fix C++11 raw string syntax group to exactly follow the standard.
Previous version had problems in delimiter definition. I've tested that raw strings with delimiter would not get correctly highlighted in Vim 7.4.283, and would segfault Vim in 7.4.205. Also, too wide character set was allowed for delimiter (non-source characters, like `@$, and whitespaces are disallowed).