Fix nested commands in interactive mode triggering prompt2 (issue #19)

Save and restore cmd_mode across interpolation parsing so that
luaX_readcommandcont resumes in the correct mode after a nested
backtick command inside ${...}.

issues/19-nested-commands-prompt2.md

@@ -1,6 +1,6 @@
 # 19 — Nested commands in interactive mode trigger prompt2
 
-**Status:** open
+**Status:** done
 
 Running nested commands in an interactive command drops into prompt2 when it shouldn't: