Runmd

Changed
* move call to valid_config() from load_config() to cli.py

Fixed
* fix error related to the --file option

Added
* add `tag` attribute
* add the possibility to run all code blocks with a given tag using `runmd run <tag>`

Changed
* rework on runner.py to print STDERR outputs in terminal
* refactor cli.py for lisibility and control

Changed
* cli and all functions in process.py have been refactored to gain in lisibility and usability

Added
* add support for environment variables
* add --file option to specifiy the markdown file to process

Changed
* change `ls` command to `list` for homogeneity

Removed
* remove --dir option since it creates too much complexity for very small benefits

Added
* add `show`option to show the code block in terminal
* show `(<lang>)` or `(<lang>: not configured)` next to the code block name
* add config file validation
* add init command and copy_config function to copy the config file to destination

Changed
* add type hints to functions
* move functions related to configuration to config.py
* move parse_markdown function to parser.py
* move run_code_block function to runner.py
* move process_markdown_files function to process.py

Fixed
* prevent running code block for not configured languages