ConnectionFileMixin is Configurable

it has `config=True` traits

Currently in master `ipython console -h` will fail because of this.
It doesn't affect 1.0, I'm not sure why.
test that `-h` and `--help-all` work for various IPython entry points

should help catch when we break these things.
fix one remaining Transformer in nbconvertapp

prevented `nbconvert -h` from working
de-decorate help output tests
only test `--help-all`

help-all is a superset of `-h`, so no need to do both,
since these tests are slow.