support for subroutine name
A little improvement of function print_keyword_sub

To colorize the subroutine's name, we expect it to be on the same
line as `sub keyword, otherwise we won't give it a shit. By the
way, I don't know if we can trust the `isblank, `isalnum, etc.,
included in `#include <ctype.h> header file.
NOTE: there is a bug, if the very first word of the input stream
is a keyword, unfortunately, it does not match. This will be
resolved later.