feat(handler): Add options.handler to handle incoming request

A function to use as main request handler (`function(req, res)`).

This provides the ability to serve the local directory. Ex:

    var finalhandler = require('finalhandler');
    var serveIndex = require('serve-index');
    var serveStatic = require('serve-static');

    tinylr({
      handler: function(req, res) {
        var done = finalhandler(req, res);
        var index = serveIndex(path.resolve('./'), {icons: true});
        var serve = serveStatic(path.resolve('./'));

        if (/changed/.test(req.url)) return this.handle(req, res);

        serve(req, res, function(err) {
          if (err) return done(err);
          index(req, res, done);
        });
      }
    });

When not defined, the default handler takes place.

lib/server.js

@@ -45,11 +45,13 @@ Server.prototype.configure = function configure(app) {
   var self = this;
   debug('Configuring %s', app ? 'connect / express application' : 'HTTP server');
 
+  var handler = this.options.handler || this.handler;
+
   if (!app) {
     if ((this.options.key && this.options.cert) || this.options.pfx) {
-      this.server = https.createServer(this.options, this.handler.bind(this));
+      this.server = https.createServer(this.options, handler.bind(this));
     } else {
-      this.server = http.createServer(this.handler.bind(this));
+      this.server = http.createServer(handler.bind(this));
     }
     this.server.on('upgrade', this.websocketify.bind(this));
     this.server.on('error', function() {

readme.md

@@ -146,6 +146,8 @@ See [gulp-livereload](https://github.com/vohof/gulp-livereload) repo.
 - `livereload`    - Path to the client side lib (defaults to `path.join(__dirname, '../node_modules/livereload-js/dist/livereload.js')`)
 - `port`          - Livereload port (defaults to `35729`)
 - `errorListener` - A callback to invoke when an error occurs (otherwise, fallbacks to standard error output)
+- `handler`       - A function to use as main request handler (`function(req,
+  res)`). When not defined, the default handler takes place.
 - `app`           - An express or other middleware based HTTP server
 - `key`           - Option to pass in to create an https server
 - `cert`          - Option to pass in to create an https server