Implement task lists

[daenvil]

• Added support for task lists (closes #4):
  • Starting an unordered list item with [ ] or [x] will create a checkbox.
  • The checkboxes are clickable by default and emit a signal when clicked.
  • There's an option to disable cliking on checkboxes.
  • There's an option to customize the checkbox.
  • See the README file in the repo for more info.
• Added support for custom syntax (closes #7):
  • Refactored the processing of all syntax elements into individual methods so they can be overridden if desired (_process_text_formatting_syntax(), _process_link_syntax(), etc.).
  • Added two empty methods: _preprocess_line() and _process_custom_syntax() which can be overridden to insert custom syntax.
• Fixed bug where a table could break all the following text.

[generrosity]

Thank you @daenvil to considering the custom syntax, and the refactoring 💚 I hope others take advantage of this new 'advanced feature' and let you know too

And awesome to see typing enforcement scattered through there too. The little autocomplete and performance is good :)