If one argument, input = output

If there was only one (non-option) argument, we used to interpret it as
a file and write the formatting result to stdout. I think the formatter
is now good enough that we can instead assume input=output in this case.
Thus, to format your entire source directory, you can simply use

    ceylon run ceylon.formatter source

(where you would previously need to give the second argument twice).

If you want to have the formatter output to stdout, use “pipe mode” (no
arguments):

    ceylon run ceylon.formatter < source/file/path.ceylon