Support custom environment variables for load_env_levels (#3327)

SPDLOG_LEVEL is currently supported to control log levels via
load_env_levels.

This patch adds support for other environment variable names, such as
MYAPP_LEVEL, for load_env_levels.

README.md

@@ -387,6 +387,9 @@ void android_example()
 int main (int argc, char *argv[])
 {
     spdlog::cfg::load_env_levels();
+    // or specify the env variable name:
+    // MYAPP_LEVEL=info,mylogger=trace && ./example
+    // spdlog::cfg::load_env_levels("MYAPP_LEVEL");
     // or from the command line:
     // ./example SPDLOG_LEVEL=info,mylogger=trace
     // #include "spdlog/cfg/argv.h" // for loading levels from argv