Added rechner plugin

plugins/rechner.lua

@@ -0,0 +1,14 @@
+function run(msg, matches)
+  -- Convert expression to a function and execute it
+  local expression = string.gsub(matches[1], "(%a+)", "math.%1")
+  return load('return '..expression)()
+end
+
+return {
+  description = "Ein simpler Taschenrechner",
+  usage = "/calc [Rechnung]",
+  patterns = {
+    "^/calc (.*)$"
+  },
+  run = run
+}