Refactored SLRE command line utility

I commented on the makefile because it was relevant there, but it might as well be relevant here :-)

Should we have a makefile rule that actually runs this code or do you plan to put it in the slre repo?

For lazymen like me who don't want to type `gcc -o /tmp/slre src/slre.c src/utf.c -DSLRE_TEST` :-)