feat(unit): Allow user to control enabling units

Fix #69 - A user may provide an `enable` attribute of a unit in their
cloud config document. If true, coreos-cloudinit will instruct systemd
to enable the associated unit. If false, the unit will not be enabled.

Fix #71 - The default enable behavior has been changed from on to off.
refactor(unit): Separate UnitDestination from PlaceUnit
feat(unit): Allow units to be enabled even if contents not provided