Solves #451 explain permalink and import tree now, and add command

README.md

@@ -31,6 +31,10 @@ Type `levels` to see the available lessons / challenges (and which ones you have
 
 For some added fun, there is a "git golf" concept where we keep track of how many commands you use to solve each level. See if you can match all of our records!
 
+### Sharing permalinks
+
+You can share a link to LearnGitBranching with an arbitrary set of commands that will execute upon load by using the `commmand` URL parameter. You will also likely want to disable the intro dialog for this case with the `NODEMO` url param; here is [an example](https://learngitbranching.js.org/?NODEMO&command=echo%20%22hello%22;%20git%20commit) to get started.
+
 ### Level Builder
 
 You can build levels with `build level`. The dialog will walk you through the process, and at the end you can `export level` to get a JSON blob. Paste that in a gist or directly into an issue and I can check it out / merge in your changes! You can also share this level directly with friends by having them run "import level" or simply specify a gist ID in the url params like so:

src/js/dialogs/sandbox.js

@@ -14,6 +14,8 @@ exports.dialog = {
         'beginner, just go ahead and start with the first. If you already know some Git basics, ',
         'try some of our later more challenging levels.',
         '',
+        'You can see all the commands available with `show commands` at the terminal.',
+        '',
         'PS: Want to go straight to a sandbox next time?',
         'Try out ',
         '[this special link](http://pcottle.github.io/learnGitBranching/?NODEMO)'

src/js/sandbox/commands.js

@@ -125,7 +125,8 @@ var regexMap = {
   'importLevelNow': /^importLevelNow($|\s)/,
   'import tree': /^import +tree$/,
   'import level': /^import +level$/,
-  'undo': /^undo($|\s)/
+  'undo': /^undo($|\s)/,
+  'share permalink': /^share( +permalink)?$/
 };
 
 var getAllCommands = function() {

src/js/sandbox/index.js

@@ -223,6 +223,16 @@ var Sandbox = Backbone.View.extend({
     });
   },
 
+  sharePermalink: function(command, deferred) {
+    var treeJSON = JSON.stringify(this.mainVis.gitEngine.exportTree());
+    var url =
+      'https://learngitbranching.js.org/?NODEMO&command=importTreeNow%20' + escape(treeJSON);
+    command.setResult(
+      intl.todo('Here is a link to the current state of the tree: ') + url
+    );
+    command.finishWith(deferred);
+  },
+
   resetSolved: function(command, deferred) {
     LevelActions.resetLevelsSolved();
     command.addWarning(
@@ -253,6 +263,7 @@ var Sandbox = Backbone.View.extend({
       'importTreeNow': this.importTreeNow,
       'import level': this.importLevel,
       'importLevelNow': this.importLevelNow,
+      'share permalink': this.sharePermalink,
     };
 
     var method = commandMap[command.get('method')];