Slash in `format` statements

[ppenzin]

Need to recognize slashes in format code. Example:

      write(*,1)
    1 format(' Double newline'//
     $       ' Followed by single newline'/
     $       ' Followed by another one'/
     $       ' A double again'//
     $       ' The end'/)
      stop
      end

Currently fort produces the following:

$ fort -fsyntax-only slash.f
../test/Sema/slash.f:3:31: error: expected ')'
    1 format(' Double newline'//
                              ^
../test/Sema/slash.f:2:15: error: use of undeclared statement label '1'
      write(*,1)
              ^