New helper: sum

#### sum
_Returns the sum of multiple numbers. Similar to `{{#add}}` block helper but accepts multiple arguments._

Parameters: `none`

**Data**
``` js
value = {
  a: 1,
  b: 2,
  c: 3
}
```
**Template**
``` html
{{sum value.a value.b value.c}}
```
**Result**
```
6
```