This PR adds support for Lean source files.

Futhermore I have slightly modified the handling of nested single-line comments. In at least Haskell and C (and also Lean) line comments are ignored if they occur inside a multi-line comment. For example the following programs both work:

{- -- -}
main = putStrLn "Hello, world!"

/* // */
#include <stdio.h>
int main(int argc, char **argv) { printf("Hello, world!\n"); return 0; }

I don't know if any other language handles this differently.