Add {{include}} template tag



#### example:

file "Dashboard/Action.html"
```html
<div class="container">
{{set $var "foo"}}
{{include "Dashboard/widget_test.html"}}
</div>
```

file "Dashboard/widget_test.html"
```html
{{$var}}
```

will output:
```html
<div class="container">
foo
</div>
```

(see code for details)

#### Note:

1. It is inline so almost does not affect performance.
2. Currently it won't check circular reference, so may cause some problems in wrong usage.