Suggestion: Template <switch> directive

Here's a suggestion to add a `<switch>` directive to the Template class. It would be used like this:
```
<switch expr="{{@myvar}}">
  <case value="123">
    ...
  </case>
  <case value="{{@anothervar}}">
    ...
  </case>
  <case value="'a string'">
    ...
  </case>
  <default>
    ...
  </default>
</switch>
```

There's one flaw in the provided code though: unquoted strings in `<case>` directives lead to an error.