Add support for loops and conditions.

Adds several new types of statements: `break`, `continue`, `if` and
`while`.

I decided to use two keywords to indicate end of an block. It's my
personal opinion that one keyword -solutions are uglier than this one:

```html
<% if 1 + 1 == 2: %>
<p>Value of <code>1 + 1</code> is indeed <code>2</code>.</p>
<% end if %>
```