command line options: allow cp-style SOURCE... DIRECTORY

ceylon run ceylon.formatter file1 dir1/file2 dir2/file3 dir means
* format file1, write to dir/file1
* format dir1/file2, write to dir/dir1/file2
* format dir2/file3, write to dir/dir2/file3
command line options: allow SOURCEDIR TARGETDIR arguments

if exactly two non-option arguments are given, and the first is a
directory, all files in the directory are formatted with output in
TARGETDIR.