Support 'ext = *' to let a formatter handle all file extensions

This is useful to configure a default formatter for all files that
applies if no other formatter can handle a file.

E.g.:

  [formatter "PLAIN_TEXT"]
    ext = *
    prio = -1

Change-Id: I4f2eb8fdf15e2abd55913f46f83071f7caa1809e
Signed-off-by: Edwin Kempin <edwin.kempin@sap.com>