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.
Whoa, so many options. I had no idea! Adding those as well.