sclang: add terminal standalone option

Add the option to tell sclang via command line option not to load any classes from the default locations in order to make it easier to make sclang based standalones

My c/c++ is very rusty, I'm not familiar with it's coding practices, and I'm not familiar with the sclang code base, so maybe the way I implemented this is not the right way to do it.  I'm also not 100% sure that change is not affecting normal supercollider use, although my intention was that if you don't pass the -a option to sclang everything should behave the same.

Running ./lang/sclang -a -l config.yaml with a yaml file pointing to a class library folder somewhere compiles the library fine and ignores everything in the default locations for extensions, etc. It also ignores the default location for a yaml file.

Also, I'm not sure if there are issues with indentation. My git gui program and Qt creator show the new code at the same indentation as the rest around it, but in github the indentation appears to be off.