Implemented following URLs from selected task.

This detects and opens only URLs with http(s) scheme, via a standard Python module webbrowser. Support for additional schemes (e.g. file://, ftp://, ...) would require writing a portable opener, or at least detecting OS with assumptions that e.g. Linux always has xdg-open installed, MacOS has open etc. Allowing users to configure that opener is a must as well.

I don't feel like doing it right now is a great idea as using webbrowser handles ~80% of all use cases for todo app, which is good enough in my opinion.