Implement ``bzr config option=value``.

* bzrlib/tests/test_config.py:
(TestWithConfigs.setUp): Slightly reorder.

* bzrlib/tests/blackbox/test_config.py:
(TestConfigDisplay): Inherits from TestWithConfigs to reduce code
duplication.
(TestConfigSet): Check the command behaviour.

* bzrlib/config.py:
(cmd_config): Implement setting a config option.
(cmd_config._show_config, cmd_config.set_config_option): Split
set/show option.