add support to load data from stdin to anyconfig_cli

[ssato]

It would be nice if anyconfig_cli can load data from stdin such like:

$ cat /etc/foo.yml | anyconfig_cli -I yaml - -O json -o -
{"a": 1, "b": [1, 2], "c": "CCC"}

[ssato]

It should work by some recent commits such as 30680e7. Here is an example.

$ cat tests/res/00-01-cnf.json
{"b": {"b": [1, 2], "c": "C"}}
$ cat tests/res/00-01-cnf.json | PYTHONPATH=. python3 anyconfig/cli.py -I json -O yaml -
b:
  b:
  - 1
  - 2
  c: C
$ cat tests/res/00-01-cnf.json | PYTHONPATH=. python3 anyconfig/cli.py -I json -O yaml - --set b.c=AAA
b:
  b:
  - 1
  - 2
  c: AAA
$