main: allow loading arbitrary config

main.lua

@@ -6,7 +6,17 @@ Node        = require('core.node')
 ThreadNode  = require('core.thread-node')
 NodeManager = require('core.node-manager')
 
-local config = require('config')
+local config = nil
+if os.getenv("SIGNAGE_CONFIG") ~= nil then
+  local f = loadfile(os.getenv("SIGNAGE_CONFIG"))
+  if f ~= nil then
+    config = f()
+  else
+    error("SIGNAGE_CONFIG given but could not be loaded")
+  end
+else
+  config = require('config')
+end
 
 local push = require('vendor.push')