[lmi] C11n: raw string literals

[Vadim Zeitlin]

 Hello again,

 I've opened a small PR introducing the use of C++11 raw string literals in
the few places where they seem to unequivocally make sense to me, please see

                https://github.com/vadz/lmi/pull/48

 Generally speaking, raw string literals seem useful to me in 2 situations:

1. Embedded regular expressions: as RE already often contain backslashes
   and are generally punctuation-heavy, not doubling the backslashes in them
   is a big gain for both readability and writeability and this is what
   this PR covers.

2. Multiline string literals: IMHO raw strings are nicer than the usual
   way of constructing such literals relying on the automatic concatenation
   of adjacent string literals as it avoids the need to escape quotes
   (which often occur in e.g. HTML text) and have literal "\n"s in the
   string. However raw strings don't allow to indent their content (unless
   you're ready to remove the extra indent at run-time) and I have no idea
   which, if any, way of formatting them would you find most aesthetically
   pleasant, so I didn't change any of them so far, please let me know if
   you'd like to discuss it further. FWIW, I'd definitely like to be able
   to use raw multiline string literals in the new code, so it would be
   nice if you could set the rules for their use.

 Thanks in advance,
VZ