#1047 Support Go's backquoted raw strings in LexCPP.cxx

[Eric Promislow]

Simple change to LexCPP: Add an option "lexer.cpp.backquoted.strings" and delimit multi-line raw strings with ... if it's true. These strings don't allow embedded backquotes, so no escaping is needed, so the SCE_C_STRINGRAW state works fine.

[Eric Promislow]

Unfortunately the backquotes were interpreted by the markdown processor.

I meant something like this:

log.Printf("%s", `first line
second line
third line`);

[Neil Hodgson]

Committed as [6e84f5].