Confluence-poster

* Bumped of marshmallow to 3.12
* Bumped atlassian-python-api version to 3.11

* Bumped and tested markdown 3.3.4 version
* Bumped tomlkit to 0.7.2
* Added tox testing for python 3.8 and 3.9
* Tested pyxdg 0.27

* Fixed file format reading from config (42)
* Added instructions to install from nitratesky

Small release that replaces [xdg](https://pypi.org/project/xdg/) with [pyxdg](https://pypi.org/project/pyxdg/)

* Added support for markdown files. Implemented as a separate command `convert-markdown` as well as a runtime conversion.
* Now the poster may be run in filter mode.

* Easier mechanism for uploading multiple files. Now it's possible to run `confluence_poster --upload-files FILE1 FILE2 FILE3` to upload them all
* Added force-create command parameter to skip prompt to create page
* Added force_overwrite config flag to allow overwriting individual pages skipping author check (like `--force` but not globally)

Started rewriting the tests to simplify them