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
fix(docs): use better group example
Merge pull request #78 from robszumski/update-user-group

fix(docs): use better group example
feat(docs): note config-drive
Merge pull request #72 from bcwaldon/unit-enable

Address unit enabling issues
Merge pull request #79 from robszumski/note-config-drive

feat(docs): note config-drive
Update cloud-config.md to include expected file format

Clarify root-level keys. Use page structure to indicate expected values.
Update cloud-config.md option descriptions

Re-word a few more things to look more like docs.
Update cloud-config.md based on feedback
Update cloud-config.md to use "parameter", not "option"
Update cloud-config.md - use "you"