add support for formatters that change the file directly and send no stdout

[hek14]

for python, we typically use black formatter, it doesn't send the formatted lines back, with this PR, we can support for this case by using fs_read. A example config can be:

      configs.python = {
        cmd = 'black',
        args = {},
        find = false,
        fname = true,
        stdin = false,
        do_not_need_stdout = true,
      }