Added an option to remove extra whitespace around hbs tags

It annoys me and apparently many others that handlebars does not strip extra whitespace around its elements when compiling. 

handlebars-lang/handlebars.js#336

This is a quick hack to make it strip that extra whitespace as if the rows with only a single hbs tag were not separate rows. This pull possibly has some bugs in its regexp, but it's a start.

I thought that maybe this should be toggleable by the user, so only by adding an option: removeHbsWhitespace:true to grunt config of assemble will enable this feature. (without it, no ill effects should happen)

With removeHbsWhitespace enabled code like this:
```
{{#each collection}}
<div>{{this}}</div>
{{/each}}
```
normally resulting this:
```
<div>ele1</div>

<div>ele2</div>

<div>ele3</div>
```

would be preprocessed to be like this:
´´´
{{#each collection}}<div>{{this}}</div>{{/each}}
´´´
resulting this:
```
<div>ele1</div>
<div>ele2</div>
<div>ele3</div>
```

This feels more natural and helps creating cleaner and more readable layouts, partials and page code.

tasks/assemble.js

@@ -27,6 +27,14 @@ module.exports = function(grunt) {
   var utils      = assemble.Utils;
   var extensions = utils.ExtensionMap;
 
+  //Removes extra whitespace around {{}}-elements somewhat like .mustache does.
+  var removeHbsWhitespace = function(assemble,filecontent){
+    if(assemble.options.removeHbsWhitespace){
+      filecontent = filecontent.replace(/\n\s*(\{\{\{[^}]+?\}\}\}\n)/gi,"$1");
+      filecontent = filecontent.replace(/\n\s*(\{\{[^}]+?\}\}\n)/gi,"$1");
+    }
+    return filecontent;
+  };
 
   grunt.registerMultiTask('assemble', 'Compile template files with specified engines', function(){
 
@@ -133,6 +141,9 @@ module.exports = function(grunt) {
 
           var partial = fs.readFileSync(filepath, 'utf8');
 
+          //If remove hbs whitespace...
+          partial = removeHbsWhitespace(assemble,partial);
+
           partial = assemble.engine.compile(partial, {
             preprocessers: [
               assemble.yamlPreprocessor(filename, function(output) {
@@ -275,6 +286,9 @@ module.exports = function(grunt) {
             grunt.verbose.writeln('compiling page ' + filename.magenta);
             var pageContext = {};
 
+            //If remove hbs whitespace...
+            page = removeHbsWhitespace(assemble,page);
+
             page = assemble.engine.compile(page, {
               preprocessers: [
                 assemble.yamlPreprocessor(filename, function(output) {
@@ -548,6 +562,9 @@ module.exports = function(grunt) {
       layout = fs.readFileSync(layout, 'utf8');
     }
 
+    //If remove hbs whitespace...
+    layout = removeHbsWhitespace(assemble,layout);
+
     var layoutData = {};
     layout = assemble.engine.compile(layout, {
       preprocessers: [