Locate output directory in project dir by default

With this change we can do something like this:

    mkdir /tmp/foo
    jbake -i /tmp/foo
    jbake /tmp/foo
    jbake -s /tmp/foo/output

All command from one place.

Other way (generate site inplace)

    mkdir /tmp/foo
    jbake -i /tmp/foo
    jbake /tmp/foo target
    jbake -s target

No more `cd` commands