ccd: take the argument from the current directory if not specified

works like ocd now, and also allows ocd and ccd to compliment each other nicely:

fedora.vm:~% ocd qtest-qml
fedora.vm:~/home:w00t:branches:CE:MW:Shared/qtest-qml% ccd
fedora.vm:~/code/qtest-qml% ocd
fedora.vm:~/home:w00t:branches:CE:MW:Shared/qtest-qml%