improve serverMiddleware

This allows resolve using ~ path nation

lib/server.js

@@ -2,6 +2,7 @@
 
 const http = require('http')
 const connect = require('connect')
+const path = require('path')
 
 class Server {
   constructor (nuxt) {
@@ -11,15 +12,29 @@ class Server {
     this.server = http.createServer(this.app)
     // Add Middleware
     this.nuxt.options.serverMiddleware.forEach(m => {
+      this.useMiddleware(m)
+    })
+    // Add default render middleware
+    this.useMiddleware(this.render.bind(this))
+    return this
+  }
+
+  useMiddleware (m) {
+    // Require if needed
+    if (typeof m === 'string') {
+      let src = m
+      // Using ~ shorthand to resolve from project srcDir
+      if (src.indexOf('~') === 0) {
+        src = path.resolve(this.nuxt.options.srcDir, src.substr(1))
+      }
+      // eslint-disable-next-line no-eval
+      m = eval('require')(src)
+    }
     if (m instanceof Function) {
       this.app.use(m)
     } else if (m && m.path && m.handler) {
       this.app.use(m.path, m.handler)
     }
-    })
-    // Add default render middleware
-    this.app.use(this.render.bind(this))
-    return this
   }
 
   render (req, res, next) {