Add Middleman detector

https://middlemanapp.com/

Author: tysongach

src/detectors/middleman.js

@@ -0,0 +1,16 @@
+const { existsSync } = require("fs");
+
+module.exports = function() {
+  if (!hasRequiredFiles(["config.rb"])) return false;
+
+  return {
+    type: "middleman",
+    port: 8888,
+    proxyPort: 4567,
+    env: { ...process.env },
+    command: "bundle",
+    possibleArgsArrs: [["exec", "middleman", "server"]],
+    urlRegexp: new RegExp(`(http://)([^:]+:)${4567}(/)?`, "g"),
+    dist: "build"
+  };
+};