Error indicator not unicode aware

[mgreter]

With latest perl-libsass we discovered two bugs in how libsass creates the error indicator (the excerpt of error line with arrow indicator). First e.pstate.src does not seems to reference the right content when parsing interpolated selectors. Second the algorithm to create the excerpt is not unicode aware, as it only counts bytes, which can lead to unicode errors if implementors have strict rules for their strings (as perl does).

I.e. the following line should use utf8::advance and similar helpers:

msg_stream << ">> " << std::string(line_beg + move_in, line_end - shorten) << "\n";

Will post a hotfix to solve the most urgent issue, since it is currently failing most of perl-libsass installs.