feat(sum): Adds a sum filter

This is the sum filter.
Hope you find this good.
If you like it we can use the same principle for statistical functions, for example average etc.

To use with markup:
 - array of numbers or parseble variables:
    markup:
```
    {{ array | sum }}
```
 - Array of objects with parameters that have numbers or parseble variables:
```
    array = [
                    {
                        name: 'Adanac apple',
                        inventory: {
                            amount: 1
                        }
                    },
                    {
                        name: 'Smith apple',
                        inventory: {
                            amount: 2
                        }
                    },
                    {
                        name: 'Annie apple',
                        inventory: {
                            amount: 3
                        }
                    },
                    {
                        name: 'Pacific apple',
                        inventory: {
                            amount: 4
                        }
                    }
                ]
```
    markup:
```
    {{ array | sum:'inventory.amount' }}
```