Look for a #! line, and return the value following the #! if one exists, or
None if this file is not a script.
I don't know if there are any conventions for quoting in Windows shebang
lines, so this doesn't support any; therefore, you may not pass any
arguments to scripts invoked as filters. That's probably wrong, so if
somebody knows more about the cultural expectations on Windows, please feel
free to fix.
This shebang line support was added in support of the CGI tests;
appropriately enough, I determined that shebang lines are culturally
accepted in the Windows world through this page:
@param filename: str representing a filename
@return: a str representing another filename.