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.
We should mention here that the format for this value is the same as the eponymous `mesos-slave` option.  That is, it can also accept non-ZK IP addresses.