Flask-sitemapper

* Adds support for dynamic Flask routes (see wiki)
* Now stores sitemap XML after the first request instead of generating it on each request
* Tests have been improved
* Documentation has been updated

This release adds support for Python `datetime` objects (as well as strings) for the `lastmod` argument, moves documentation from `README.md` to the GitHub wiki, and adds project URLs to the package metadata.

Main Changes in This Version
* Now fully supports apps using multiple domains, resolving issue 2
* Uses pre-defined arguments instead of `**kwargs` for lastmod, changefreq, and priority.
* Adds functionality to serve sitemaps with GZIP.
* Improved code comments, docstrings, and spelling.
* Improves and updates `README.md`

Main Changes in This Version
- Refactors the project
- Changes build system to Poetry
- Adds tests
- Improves README
- Adds option to gzip sitemaps
- Improves code comments
- Adds documentation for developers/contributors

**WARNING:** From this version onwards, Flask Sitemapper requires Python >=3.7

Version 1.4.0 was deleted shortly after release due to an incorrectly named `__init__.py` which meant the version could not be imported properly. Checks will be more thorough and versioning will be more consistent from now on.