Multicommand

* **Update**: `load_parsers` now uses `pkg.__name__` when loading parsers into the parser registry (OrderedDict). Prior to this, the value `commands` was hardcoded meaning that for multicommand to work the subpackage housing all of the parsers __had__ to be called `mypkg.<blah>.commands`.
* **Update**: Documentation
* General housekeeping: Fixed typos, fixed broken links, added PyPI badge, etc.

- **Fix:** Fixed the scenario where "intermediate" commands were not defined. When that happened multicommand wouldn't have intermediate parsers to link terminal parsers to the root parser.

Specifically, if there was say a single command defined as: `commands/mytopic/mycommand/mysubcommand.py` (with appropriate `__init__.py` files) `multicommand` would crash, because the index parsers required to exist "between" the parser defined in `mysubcommand.py` and the root parser (created by multicommand) weren't getting created, and the application would crash.

This release also adds a _very_ basic README and a [simple example](https://github.com/andrewrosss/multicommand/tree/master/examples/01_simple) demonstrating the basic usage of `multicommand`.

* **Update:** Forward parser config on subparser creation
* **Update:** Add custom `prog=...` when linking parsers to accurately reflect the expected command invocation
* **Maintenance:** Add comments, clean up type hints, update `pyproject.toml`

Hello, World!