Cmd2

* Bug Fixes
* Fixed issue where subcommand added with `as_subcommand_to` decorator did not display help
when called with `-h/--help`.
* Enhancements
* `add_help=False` no longer has to be passed to parsers used in `as_subcommand_to` decorator.
Only pass this if your subcommand should not have the `-h/--help` help option (as stated in
argparse documentation).

* Bug Fixes
* Fixes an issue introduced in 1.3.0 with processing command strings containing terminator/separator
character(s) that are manually passed to a command that uses argparse.

* Breaking changes
* The functions cmd2 adds to Namespaces (`get_statement()` and `get_handler()`) are now
`Cmd2AttributeWrapper` objects named `cmd2_statement` and `cmd2_handler`. This makes it
easy to filter out which attributes in an `argparse.Namespace` were added by `cmd2`.
* Deprecations
* ``Namespace.__statement__`` will be removed in `cmd2` 2.0.0. Use `Namespace.cmd2_statement.get()`
going forward.

* Bug Fixes
* Fixed `RecursionError` when printing an `argparse.Namespace` caused by custom attribute cmd2 was adding
* Enhancements
* Added `get_statement()` function to `argparse.Namespace` which returns `__statement__` attribute

* Bug Fixes
* Fixed `AttributeError` when `CommandSet` that uses `as_subcommand_to` decorator is loaded during
`cmd2.Cmd.__init__()`.
* Enhancements
* Improved exception messages when using mock without `spec=True`.
See [testing](https://cmd2.readthedocs.io/en/latest/testing.html) documentation for more details on testing
cmd2-based applications with mock.

* Breaking changes
* CommandSet command functions (do_, complete_, help_) will no longer have the cmd2 app
passed in as the first parameter after `self` since this is already a class member.
* Renamed `install_command_set()` and `uninstall_command_set()` to `register_command_set()` and
`unregister_command_set()` for better name consistency.
* Bug Fixes
* Fixed help formatting bug in `Cmd2ArgumentParser` when `metavar` is a tuple
* Fixed tab completion bug when using `CompletionItem` on an argument whose `metavar` is a tuple
* Added explicit testing against python 3.5.2 for Ubuntu 16.04, and 3.5.3 for Debian 9
* Added fallback definition of typing.Deque (taken from 3.5.4)
* Removed explicit type hints that fail due to a bug in 3.5.2 favoring comment-based hints instead
* When passing a ns_provider to an argparse command, will now attempt to resolve the correct
CommandSet instance for self. If not, it'll fall back and pass in the cmd2 app
* Other
* Added missing doc-string for new cmd2.Cmd __init__ parameters
introduced by CommandSet enhancement