Statements like `if` and `unless` are implemented as built-in helpers. Helpers
can be invoked three ways, each of which is illustrated below with conditionals.

The first style of invocation is **inline invocation**. This looks similar to
displaying a property, but helpers accept arguments. For example:

```handlebars
<div>
  {{if isFast "zoooom" "putt-putt-putt"}}
</div>
```

`{{if}}` in this case returns `"zoooom"` when `isFast` is true and
`"putt-putt-putt"` when `isFast` is false. Helpers invoked as inline expressions
render a single value, the same way that properties are a single value.

Inline helpers don't need to be used inside HTML tags. They can also be used
inside attribute values:

```handlebars
<div class="is-car {{if isFast "zoooom" "putt-putt-putt"}}">
</div>
```

**Nested invocation** is another way to use a helper. Just like inline helpers,
nested helpers generate and return a single value. For example, this template
only renders `"zoooom"` if both `isFast` and `isFueled` are true:

```handlebars
<div>
  {{if isFast (if isFueled "zoooom")}}
</div>
```

The nested helper is called first returning `"zoooom"` only if `isFueled` is
true. Then the inline expression is called, rendering the nested helper's
value (`"zoooom"`) only if `isFast` is true.

The third form of helper usage is **block invocation**. Use block helpers
to render only part of a template. Block invocation of a helper can be
recognized by the `#` before the helper name, and the closing `{{/` double
curly brace at the end of the invocation.

For example, this template conditionally shows
properties on `person` only if that it is present:

```handlebars
{{#if person}}
  Welcome back, <b>{{person.firstName}} {{person.lastName}}</b>!
{{/if}}
```

`{{if}}` checks for truthiness, which means all values except `false`,
`undefined`, `null`, `''`, `0`  or `[]` (i.e., any JavaScript falsy value or an
empty array).

If a value passed to `{{#if}}` evaluates to falsy, the `{{else}}` block
of that invocation is rendered:

```handlebars
{{#if person}}
  Welcome back, <b>{{person.firstName}} {{person.lastName}}</b>!
{{else}}
  Please log in.
{{/if}}
```

`{{else}}` can chain helper invocation, the most common usecase for this being
`{{else if}}`:

```handlebars
{{#if isAtWork}}
  Ship that code!
{{else if isReading}}
  You can finish War and Peace eventually...
{{/if}}
```

The inverse of `{{if}}` is `{{unless}}`, which can be used in the same three
styles of invocation. For example, this template only shows an amount due when the
user has not paid:

```handlebars
{{#unless hasPaid}}
  You owe: ${{total}}
{{/unless}}
```
