New <!if> tag

```
<!if condition...>
  ...
<!elseif condition...> 
  ...
<!else>
  ... 
<!end>
```

Design notes:

1. `<!elseif>` or `<!elif>`?
2. How the *condition* will work? Are we going to handle variables directly? E.g.
   ```
   <!set a 1>
   <!if a == 1> ... <!end>
   ```
   or 
   ```
   <!if <!get a> == 1> ... <!end>
   ```