Make --watch deal with configuration changes.

This change means that you can run `bundle / build --watch` and have it
respect config changes.

If in tab one you have:

  jspm bundle app --watch

And then in another you run:

  jspm install jquery

When you change your app to `import $ from jQuery` it will pick up the
change and include jQuery in the generated bundle. Previously at this
point it would error and complain that it couldn't find jQuery, because
the config hadn't been reloaded.

lib/bundle.js

@@ -21,6 +21,7 @@ var fs = require('fs');
 var asp = require('bluebird').Promise.promisify;
 var extendSystemConfig = require('./common').extendSystemConfig;
 var toFileURL = require('./common').toFileURL;
+var JspmSystemConfig = require('./config/loader').JspmSystemConfig;
 
 function camelCase(name, capitalizeFirst) {
   return name.split('-').map(function(part, index) {
@@ -149,7 +150,9 @@ exports.bundle = function(moduleExpression, fileName, opts) {
   opts = opts || {};
   fileName = fileName || path.resolve(config.pjson.baseURL, 'build.js');
 
-  function bundle() {
+  function bundle(builder) {
+    var latestBuilder = builder || systemBuilder;
+
     return Promise.resolve()
     .then(function() {
       if (!opts.sourceMaps)
@@ -158,7 +161,7 @@ exports.bundle = function(moduleExpression, fileName, opts) {
     .then(function() {
       ui.log('info', 'Building the bundle tree for %' + moduleExpression + '%...');
 
-      return systemBuilder.bundle(moduleExpression, fileName, opts);
+      return latestBuilder.bundle(moduleExpression, fileName, opts);
     })
     .then(function(output) {
       logTree(output.modules);
@@ -218,7 +221,9 @@ exports.build = function(expression, fileName, opts) {
   opts = opts || {};
   fileName = fileName || path.resolve(config.pjson.baseURL, 'build.js');
 
-  function build() {
+  function build(newBuilder) {
+    var latestBuilder = newBuilder || systemBuilder;
+
     return Promise.resolve()
     .then(function() {
       if (!opts.sourceMaps)
@@ -227,7 +232,7 @@ exports.build = function(expression, fileName, opts) {
     .then(function() {
       ui.log('info', 'Creating the single-file build for %' + expression + '%...');
 
-      return systemBuilder.buildStatic(expression, fileName, opts);
+      return latestBuilder.buildStatic(expression, fileName, opts);
     })
     .then(function(output) {
       logTree(output.modules, output.inlineMap ? output.inlineMap : opts.rollup);
@@ -299,7 +304,7 @@ function createWatcher(files, opts) {
 
   var watcher;
   // share the existing watcher if possible
-  if (watchman && lowestDir != watchDir || 
+  if (watchman && lowestDir != watchDir ||
       !watchman && (lowestDir != watchDir || JSON.stringify(watchFiles) != JSON.stringify(relFiles))) {
     var sane = require('sane');
     if (curWatcher)
@@ -360,6 +365,12 @@ function buildWatch(builder, output, build) {
       return path.resolve(config.pjson.baseURL, file);
     });
 
+    var configFiles = [config.pjson.configFile];
+    if (config.pjson.configFileBrowser) configFiles.push(config.pjson.configFileBrowser);
+    if (config.pjson.configFileDev) configFiles.push(config.pjson.configFileDev);
+
+    files = files.concat(configFiles);
+
     var unwatch = createWatcher(files, {
       ready: function(dir, watchman) {
         ui.log('info', 'Watching %' + (path.relative(process.cwd(), dir) || '.') + '% for changes ' + (watchman ? 'with Watchman' : 'with Node native watcher') + '...');
@@ -377,7 +388,15 @@ function buildWatch(builder, output, build) {
 
     var building = false;
     var rebuild = false;
+    var changeWasConfigFile = false;
     function changed(file) {
+      changeWasConfigFile = ( configFiles.indexOf(file) > -1 );
+
+      if (changeWasConfigFile) {
+        config.loader = new JspmSystemConfig(config.pjson.configFile);
+        builder = new Builder();
+      }
+
       if (file) {
         builder.invalidate(toFileURL(file));
         // also invalidate any plugin syntax cases
@@ -391,17 +410,26 @@ function buildWatch(builder, output, build) {
 
       building = true;
       rebuild = false;
-      if (file) 
+      if (file)
         ui.log('ok', 'File `' + path.relative(process.cwd(), file) + '` changed, rebuilding...');
       else
         ui.log('ok', 'File changes made during previous build, rebuilding...');
-      build().then(function(output) {
-        return buildWatch(builder, output, build);
-      }, function(err) {
-        ui.log('err', err.stack || err);
-        return buildWatch(builder, output, build);
-      })
-      .then(function(newWatcherRefresh) {
+
+      var buildPromise;
+      if (changeWasConfigFile) {
+        // don't bother doing a rebuild for a config file change
+        // but just keep watching so the next time a file changes
+        // the bundle is rebuilt but with the new Builder & config
+        buildPromise = buildWatch(builder, output, build);
+      } else {
+        buildPromise = build(builder).then(function(output) {
+          return buildWatch(builder, output, build);
+        }, function(err) {
+          ui.log('err', err.stack || err);
+          return buildWatch(builder, output, build);
+        });
+      }
+      return buildPromise.then(function(newWatcherRefresh) {
         unwatch();
         if (rebuild)
           newWatcherRefresh();