Prompt for dimmability; record it in metrics.json

y / n / Enter-if-unknown at the station; only known values are
written, and the scaffolded device page gets a Dimmable spec.

lamp_export.py

@@ -181,6 +181,8 @@ def write_bundle(reading, out_dir, meta, n_readings, supply=None):
     if meta.get("type"):
         # Lamp technology: led, halogen, cfl, incandescent, ...
         metrics["type"] = meta["type"]
+    if meta.get("dimmable") is not None:
+        metrics["dimmable"] = meta["dimmable"]
     if meta.get("variant"):
         metrics["variant"] = meta["variant"]
     if meta.get("rated"):