offer: allow offers in other currencies if we can convert.

This avoids a footgun where they create an offer then we can't create
the invoice because they don't have a converter plugin.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

tests/plugins/currencyUSDAUD5000.py

@@ -0,0 +1,18 @@
+#!/usr/bin/env python3
+"""
+This plugin is used to test the currency command
+"""
+from pyln.client import Plugin, Millisatoshi
+
+plugin = Plugin()
+
+
+@plugin.method("currencyconvert")
+def currencyconvert(plugin, amount, currency):
+    """Converts currency using given APIs."""
+    if currency in ('USD', 'AUD'):
+        return {"msat": Millisatoshi(round(amount * 5000))}
+    raise Exception("No values available for currency {}".format(currency.upper()))
+
+
+plugin.run()