The editor does support both bold and underline text.
You can edit the style for a particular scheme by going to Tools > Options > Fonts and Colours.
If you want such features in text documents you may have a bit of a problem. The editor can of course support these features but text files do not support these features so everything will be lost when the editor window is closed. You could add markup features and use PYPN to parse the text for the markups and make the changes.
Keep in mind that the markup would still need to be visible which might be distracting. If you wanted to hide them then you would need another PYPN script to re-insert them before the file was saved.
All in all it would probably be better to just use a different editor for such needs but it might be fun to implement all the same.