fix: use Nan::Utf8String instead of v8::String::Utf8Value (#38)
fix: use Nan::Utf8String instead of v8::String::Utf8Value (#38)

This fixes another incompatibility problem with node v12.0.0:

```
node_modules\diskusage\src\main.cpp(26): error C2440: '<function-style-cast>': cannot convert from 'v8::Local<v8::Value>' to 'v8::String::Utf8Value' [C:\Users\User\Documents\tmp\node_modules\diskusage\build\diskusage.vcxproj]
  c:\users\User\documents\tmp\node_modules\diskusage\src\main.cpp(26): note: No constructor could take the source type, or constructor overload resolution was ambiguous
```
jduncanator/node-diskusage,jduncanator/node-diskusage,jduncanator/node-diskusage