Theme system

This is a slightly more complicated logger than what is done for lektor.
The process to create a logger is the following:

{ // In scope
std::shared_ptr<LogSink> mySink   = VIVY_NEW_LOG_SINK();
std::shared_ptr<Logger> appLog    = VIVY_GET_LOGGER(mySink, "APPLICATION");
std::shared_ptr<Logger> scriptLog = VIVY_GET_LOGGER(mySink, "SCRIPT");
// ...
VIVY_LOG_WARN(appLog) << "This is" << ' ' << "very usefull!";
// ...
} // Will clear all loggers