Avoid creating `std::string` object when not required

[artemp]

trafficstars

#define TOSTR(obj) (*v8::String::Utf8Value((obj)->ToString()))

Would it make sense to not always create std::string from char const*? For example

p.parse(TOSTR(info[0]); 

^ this call can be expensive as info[0] represents SVG.

May we should add an extra helper macro which returns char const*

#define TO_CSTR(obj) (*v8::String::Utf8Value((obj))

Or explore using string_view like objects?

/cc @springmeyer

[springmeyer]

Nice catch. Yes, we should move to using Nan::Utf8String consistently per https://github.com/mapbox/node-cpp-skel/issues/54