Stuffpages

* new value in config.py: `ignore_dirs`
* lifted restrictions on where `output_dir` can be in relation to input directory
* ask for confirmation when `output_dir` contains input directory

* StuffPages can now be installed via pipx

* Fix for not copying `config.py` when installing
* Dark theme added
* Documentation fixes

- Fix for favicon in config
- Fix for relative links
- Fixes for Windows paths

- new template system (string.Template) using `$variable` notation instead of `{{variable}}`
- `output_dir` is now relative to `_stuffpages` directory
- internal links fixes
- html output fixes
- default theme CSS improvements and fixes
- 4 new themes: selenized-light, selenized-dark, selenized-white, selenized-black
- switched to different code highlighting plugin (pymdownx.highlight)
- updated config default settings

* Small fixes
* CSS improvements (figure, figcaption, table, iframe)
* Examples update