Argument convension as helpers

Would be nice to pass parametres like `style`, `required` and customs for making reusable widgets.
Added some to the example:
```html
{{{ContinuouslySavingTextArea "entry" value=entry style="color: blue;" required="required"}}}
```
Widget:
```html
<template name="ContinuouslySavingTextAreaView">
  <textarea class="textbox" style="{{style}}" {{required}}></textarea>
  <br/>
  Current value: {{value}}
</template>
```