Jestspectation

* Create [documentation site](https://miguelguthridge.github.io/Jestspectation/).
* Fix various documentation issues.
* Export `JestspectationBase` to allow others to create custom matchers more easily.

* `ObjectContainingProperties` to match objects with properties, requiring them to exist
* `ObjectContainingItems` to match objects with properties, requiring them to equal given values
* When using Pytest integration with no Jestspectation base classes, format diffs as `actual == expected` rather than `expected == actual`

* Switch to Poetry as a dependency manager
* Enable pytest plugin by default
* Fix compatibility with Python 3.9

* Fix a bug with LinesMatch oops

* Add `LinesLike` for matching TextLike, except with better diffs for multi-line strings
* Add option to use Jestspectation for all Pytest diffs
* Refactoring

Improved documentation