Complete docs for command line flags

* Split flags into required and optional. Currently only `--master` is
  required.
* Alphabetize the list of optional flags for easier scanning.
* List current defaults for all options as defined in
  `MarathonConf.scala`.
* Add missing options “artifact_store” and “reconciliation_interval”.

Addresses part of issue #432.
Missing: non-deprecated options defined in `ZookeeperConf.scala`: `zk_timeout`, `zk`, `zk_max_versions`.