Markdown-spa

Fixed faulty default Github workflow: wrong upload path (`./doc/generated` instead of `./generated`)

Added an optionnal full trackback option (`-ft` or `--full-traceback`) for commands:
- `markdown_spa build`
- `markdown_spa watch`
- `markdown_spa install`
- `markdown_spa init`
- `markdown_spa add`

- Pages can now be HTML files (rendered as standalone template)
- Added a `--upgrade` flag for `markdown_spa install`

- Fixed echoing "... failed" even if copying blank dir was successful
- Using `click.secho` instead of custom `echo` function

- Fixed default SASS configuration
- Blank project files are included when running `pip install markdown_spa `
- Blank project creation is faster
- `mardown_spa init` isn't git dependent anymore
- Added `mardown_spa list` command to list all installed extensions

Using `{sys.executable} -m pip` instead of `pip install` for compatibility