Paste these into Script Editor and save them as "Markdown Italicize.scpt" and "Markdown Boldise.scpt" in SubEthaEdit's Script folder (find it by opening it from the Script menu).
on seescriptsettings()
return {keyboardShortcut:"@i", shortDisplayName:"Markdown Italicize", displayName:"Markdown Italicize"}
end seescriptsettings
tell application "SubEthaEdit"
begin undo group
set thetext to the the contents of the selection