HTML tag support is aligned with Mastodon 4.2 supported tags.
This code introduces a soft dependency on the urwidgets library.
If urwidgets is not available, HTML tags are still supported,
but hyperlinks are not underlined using the OCS 8 terminal
feature (on supported terminals).
The instance domain can be different from their base url, for example
the instance at https://social.vivaldi.net uses the vivaldi.net domain,
sans 'social'.
This commit requires the user to provide the base url of the instance,
instead of domain name. The domain is then fetched from the server.
fixes#217
