Break and continue tags

Fixes #100. Here is the change summary:

**New exception**: `InterruptException` - this allows interruptions to be sent from anywhere. This exception, and any other that inherits from it, will not be caught by the template rendering, no matter if `RethrowErrors` is set to `True` or `False`.

**New tag**: `break` - this will act as a normal `break` statement inside a `for` loop, interrupting its execution.

**New tag**: `continue` - this will act as a normal `continue` statement inside a `for` loop, interrupting the current iteration and continuing to the next.

Both `break` and `continue` tags, if used outside of a `for` loop, wiil abort the entire rendering, without any errors.