This plugin adds text capitalization options to the Markdown editor. In the menu Edit --> Capitalization you will find 6 commands that can be applied after selecting text:
This is great. I actually have this in my backlog as well but haven't done it yet.
Do you mind adding two more commands for East Asian languages so I don't have to create a slightly different plugin? They are
Fullwidth to Halfwidth characters ( Word -> word )
Halfwidth to Fullwidth characters (word -> Word)
I can do a pull request sometime later if you're not familiar with these problems.
Also I think it will be better if the selection is kept after the conversion. What do you think?