Implement 'echo' command.

* bzrlib/tests/test_script.py:
(TestCat.test_cat_input_to_file): Test appending.
(TestEcho): Test 'echo' command.

* bzrlib/tests/script.py:
(_scan_redirection_options): Factor out redirections handling.
(TestCaseWithScript._read_input): Factor out input handling.
(TestCaseWithScript._write_output): Factor out output handling.
(TestCaseWithScript.do_cat): Simplified.
(TestCaseWithScript.do_echo): New 'echo' command, more natural
than 'cat'.