Exports quote and unquote functions for C bindings

Needed to exchange strings with registered custom functions.

Libsass passes strings in quoted form inclusive quotemarks to the C bindings. I guess this is also how it is handled internally (I see a lot of unquote stuff going on in functions.cpp). IMO it would have been better to unquote on reading and quote on output if needed. Probably too late now. So this should be handy for all C Bindings if they have to consume or return strings in custom functions.